Janos Konya
Member
In the forums we can read relevant posts in proportion to the professionals in real life, the rest is only partial or inaccurate information. (simply gossip, half true or just inaccurate - perhaps acceptable in some circumstances)
Fact:
- For ADS-B reception, a resonant antenna at 1090 MHz must be used.
The higher the frequency at which we scale an antenna, the more accuracy is required. In our case, a deviation of some less than a millimeter can separate success from failure. For complex antennas, these errors not only add up, they can cause exponential deviation.
1. / Wavelength for 1090 MHz
If the inaccuracy given by rounding is allowed, the quarter wavelength is 68 mm. (If we want to be more accurate then 68.8 mm) But! This is only true when radio waves measured in the air. In different materials, radio waves travel different lengths in a given time. The ratio that shows the degree of deviation is the Velocity Factor (VF). This number must be multiplied by the calculated value to obtain the antenna parameter. For copper and aluminum this is 0.95 (95%). For the materials we use, this means 65.3 mm. We can look for the data of the coax cables on the manufacturer's data sheet, where we can also find the VF value for them. (There are antenna parts that often can be made of coaxial cable only. For example: balun --> BALanced-UNbalanced transformer)
Our antennas are always made of a material other than air, this is why we can never use a 68 or 69 mm lenght as a quarter wavelength for cutting!
The formula we use for getting the full wavelenght:
Speed of light / frequency in MHz multiplied by the Velocity Factor (VF)
300 / 1090 x 0.95
--> If you are not interested in details, skip this part and continue reading from the next paragraph.
In the case of microwaves, the skin effect only works up to a thickness of a few microns on the surface of the conductor, but the ratio between the length and diameter of the conductor is already significantly smaller. As I mentioned, in our case the VF value for copper is 0.95 (95%) which can decrease to about 0.94 at the highest frequencies even if using a thick conductor.
Careful work at higher frequencies is much more important than in other cases. Due to the skin effect, a high-gloss, preferably the perfectly smooth surface is necessary at the highest microwave frequencies. If we have an imperfect surface, parasitic phenomena may appear near the surface. Even really small gaps and scratches can detune the VF and other parameters.
2. / Quarter wave antenna (ground plane, cantenna and the like)
We clarified above that the quarter-wave radiator of the correct size is 65.3 mm (copper wire). The center of the Spider antennas are usually N or F connector - or something similar. The length of the driven element (pin in the middle) must be measured from the point where it emerges from the connector. Try to aim for an inaccuracy of less than one percent.
An important part of the antenna is the additional "pieces of wire" or sheet metal (whatever its shape) placed around the driven element - these wires also must be resonant at the given frequency. Do you remember? - 65.3 mm and 1090 MHz The best is cutting close to the aimed lenght but not over, then using a fine emery or nail file for the last tenths of millimeter. Without caliper, 65.5 mm is ok.
Almost everyone makes mistakes here:
- If the metal shield of the connector is visible above the joints of the wires around, the visible part must also be included in lenght of gound plane elements. The shield seen there also an active part of the antenna! The threaded part of the F extensions are almost always visible, but this is not so bad, at all. (mainly when working without serious measuring devices) The visible part above the radials can also be considered as a temporarily adjustable element so that we can test the direction of change before deciding about cutting (or not) the next tiny part of the wires or the metal plate... Remember? Due to a 1-2% difference, things can go in the trash.
You could say: Mobil antenna owners never cut the roof of the car around the antenna! It is true, but they use antennas with hidden tuning elements in their antennas to tolerate the the differences of the given surface. Mobile antennas always have compromises and they are far from ideal.
- For Cantenna owners: I recommend the fact that from the protruding connector to the open end down below at the bottom of the antenna - "the entire lenght of the can plus the curved bottom from its center" - should be included in the measurement, not just the downward bent part! (Imagine a 1 meter circular plate where only the last millimeter is bent downwards ... Don't you think that only that 1 millimeter counts?
From imagination back to the real world: --> The total electrical length affects the operation.
I should mention here that the input impedance of SDR receivers can be of two types. The older ones - still fitting the TV - are 75 ohms, the newer redesigned ones are the 50 ohms (with SMA or BNC connector). We'll get back to this subject soon.
The antenna rods of ground plane are always shown in the images when bent 45 degrees down. Not by accident, but it is not a mandatory rule for all. Amateur radios use 50-ohm input, auxilary devices, and feed lines to avoid signal loss and/or equipment failure. When feed line and antenna have other values than 50 ohm, operators use matching technics. (antenna tuner, balun, unun, choke, etc.) All the components have the same impedance - and where there is a discrepancy, they are made identical to the connected component by tuning or tricky fitting. Sometimes these methods have limitations and losses but in those circumstances they are acceptable and are within the safe value range...
- For owners of a 75-ohm SDR receiver, I recommend using standard 75-ohm coaxial cables - these have better parameters than the same-priced 50-ohm coaxial cables, and last but not least, the 75-ohm dongle fits this kind of cable. There is only one more thing that needs to be changed on the antenna. Bend the ground plane wires horizontally (90 degrees from the driven element) or use a flat, horizontal plate with 130.6 mm diameter instead. (131 is ok, since it is within 1%) Do not forget adding the visible part of the shield (height) of the connector above the plate. Now, you have a matching 75 ohm antenna.
By bending the wires 45 degrees down, the antenna will be 50 ohm. When fully folded down, it is about 32.5 ohm, but in our case it cannot be used.
The radials as ground plane serve and perform several functions. In addition to being a resonant part of the antenna due to its length, it matches the impedance to the cable at the set angle and performs the function of decoupling. (Explains to the antenna that the cable is no longer part of the antenna itself.) ... and also affects the radiation pattern.
Ground plane radials are actually elements converting capacitance to inductance and vica versa. This is already a topic in specialist books and quite boring ...
3. / Franklin collinear antenna - sometimes called vertical centerfed (2, 3 or) 4 half wave
Of the wire antennas, this can even be created without using expensive instruments. Franklin antennas usually have 2 or 4 half-wave radiators. The latter is already a hard nut to crack without at least an antenna-tester or a nano-VNA on the table - but possible to create with a mountain of patience.
The half-wave radiators are 131 mm long, and the phase-matching U-elements are also made of 131 mm wire. Squeezing the "legs" of the U little by little, the frequency decreases but the coupling strengthens. Radiation pattern and gain of the antenna depends also on distance of the neighbouring active elements. So, do not chose a too small distance between U legs! This construction does not allow you to pick the ideal distances (0.7-0.9 WL) between Imax points of halfwave elements for achieving the max gain, thus we have to get the antenna resonant at 1090 MHz with the proper phase delays between radiators, at least.
In the case of half-wavelength elements, the distance between the current-maximum points (the middle of the half-wavelength parts) can start from a minimum distance of 0.5 when the distance between the endpoints is Zero - because we can find a quarter-wave part both downstream and upstream of the center of the elements.
The bigger the distance than 0.5 -> the greater the distance between the endpoints of half-wave elements.
The common theory saying that by doubling the number of half-wave elements we will have an extra 3 dB gain is not always true. Most of the times it is NOT fulfilled. Gain is also a function of the antenna design, the distance from ground surface, and the conductivity of the ground.
You can read from the graph below that a Franklin 4 element antenna with its small endpoint distances - with a bit more than 0.5 value on the x axis - can have 7dBi omnidirectional gain, but it is far from the optimal 8.8 dBi gain.
...graph shows dBd - gain above a single dipole...
Because of the sensitivity of the construction, take care of the radiators being in straight line by creating really precise bendings and fixing them to a common support stick (made of a neutral material like wood or fiber glass). The joints on the supporting stick shall not be fully fixed while you are still working on the antenna. (tune it really carefully since a part of a milimeter also counts)
The U element in the middle of the antenna is the feed point to join the coax cable. Using small "2-pin wire joint with tiny screws" on the legs of the feed point (do not forget sliding it onto the legs before bending the radiators) makes it possible to find the 50 or 75 ohm point without soldering again and again. Be patient, since you have to change the coax position little by little to find the optimal place. By tapping on different points on the stub, tuned frequency slightly changes as well. ...a little trick to make the process more comfortable: The factory made joint of the 2-pin connector must be cut and unnecessary parts removed to keep the ability of tuning the distance of the middle U legs.
For compensating the frequency shifts, all of the U parts should be finely and evenly tuned at the same time. In practice, the two phasing U on the sides have to be symmetrical, at least. So, you have to maintain the distance of the legs on 3 U elements and also the position of your coax on the center one. Any of the mentioned points on the antenna can mess up the final result. Make sure that the path of the coax leaves the antenna perpendicularly - do not lead it in parallel with the radiators and the matching stub (middle U element) before it is farther than a full wavelenght if possible. (from 28-30 cm, the distance is safe) Joining an extra wooden stick to the middle of the support stick already used for the radiators will help. They form a T.
/// I have a nano-VNA but it was still not so simple to match and tune this antenna. At the end, it had 50.23 ohm and 1.07:1 VSWR @1090 MHz, (29.4 dB return loss) ///
(note: PVC also detunes your antenna, avoid the use of it as material of suppport element)
4./ Loss of coaxial cable
There are ways to decrease or to get rid of cable losses. Use low loss coax at the impedance of your receiver and use Low Noise Amplifier (LNA) to compensate the expected loss. Amplifier shall be used at the antenna end, since the already lost or non existing signal can not be amplified at the receiver side. Use of LNA is more important if you want to use also a filter. These filters are good to keep the unwanted signals overdriving the receiver away - but filters have a so called insertion loss...
You can forget cable losses by not using the coax cable at all, or by use of a really short one (pigtail). Not a joke. These days, excelent USB extension cables are available.With a 10m lenght (30 feet) you do not necessarily have to insert extra power source for your dongle. The extension cable must be a USB2 compatible one. Beware of the ones for keyboards and mice only, because these cables have very low data rate, not like your dongle. (indoor, upstairs or attic) Connecting the antenna directly to the dongle is best. This way, you can forget bothering with coax cables. Cons: USB extension cables have the USB male plug and also an even bigger female end fixed on the cable. This is why you have to chose between drilling a huge hole - or cutting the factory-made cable to use a normal hole through the wall, and solder the previously cut cable back... I do not have to mention that this latest is against the garantie.
5./ CoCo (coaxial collinear) antennas
My opinion strictly... It is a time wasting faulty construction, mainly at 1090 MHz. There are too many cuts of elements for gathering the mistakes and almost impossible to set the necessary parameters even with Vector Network Analyzer. I have never seen a plan of coco antenna where both decoupling and matching were set correctly. There are really few documentation or study about coco antennas. Not by chance. It is really cheap and told to have so high gain, then why the hell we can not buy one in the shops? Let's see...
Intermezzo: Maniacs build them with 12 and 20 elements LOL
My opinion: Above 8 elements the extra gain is almost nothing compared to the problems we may get. Let's say, we can get it work as expected. A 8 or more element collinear has a really sharp plate-like radiation pattern, so we would lose the planes above us, plus any deviation of the antenna from the real vertical would cause even more losses. The round-plate-like pattern would cut into the ground too early - instead of catching planes in the air. The previously mentioned 1% tolerance is not acceptable here, since 1% shorter elements would down-tilt the lowest lobe of the radiating pattern, 3-4 degrees below the horizon. (it is ok if you run a repeater station on the top of a hill and you want to serve the ones in the walley. This down-tilt feature can be set easily at lower frequencies, dealing with longer elements)
So, this antenna tuned to either a bit lower or higher frequency would distort the radiating pattern. With the gain of 10+ elements you can not compensate the possibly distorted radiating pattern, the bad SWR and the mismatch of the antenna. Those who say the opposite tend to forget that the measured bad SWR at the antenna and/or the missing decoupling will always cause more loss at the other end of the coax, and the experienced loss is not linear.
(The values measured at the cable end seem to be more favorable than the real ones, but we cannot read the real properties of the antenna there. The loss of the cable also attenuates the amplitude of the reflected waves, so on a high loss and/or long cable we can see parameters close to perfect even if in reality the antenna is unusable.)
Note: Just pushing the coax pieces together is not antenna making, at all...
Assuming that the antenna is already matched correctly and the common currents of antenna and feedline are decoupled - we still have problems.
The phase shift calculated by the velocity factor of the coax cable is fine, this will force the outer surface of the adjacent antenna elements to radiate - even though they are NOT actually resonant at the calculated frequency due to their physical length. Braid radiates to air, so we should use another VF value here --> but we can not. The measured cable pieces can not be matched for both dielectric materials at the same time.
In summary: Though it can work after a lot of engineering job, but its effectiveness is questionable.
I admit, I also made some of this kind, but when I measured the parameters I would have liked to cry. I sent the final products to the trash.
---------
---------
A really well made, 4 active element antenna will almost always work better at the same site, on the same position. You can achieve greater than 8 dBi gain, at 2° just above the horizon - so keeping the antenna vertically(!) is important, since 1-2 degree of inaccuracy may result in failure.
If you set the antenna to the side of a mast, you can "tune" the gain caracteristics by picking a side-distance from 1/4 to some more than 1/2 wavelenght. At 1/4, it is almost round-like with some loss towards the mast and some more gain in other directions. At half wl, you will get the maximum 9 dBd (~11dBi) gain towards the sides. This is more than enough for an omni antenna.
see: Stacked-phased antennas... You can expect 6dBd (8dBi) of average gain.
----------
By the way, collinear antennas are one of my favourites --> they deserve some attention mainly in vertical use.
...The whole paragraph about CoCo antennas is my personal opinion. You can still love them.
The commercially made, really good antennas are never coco-s, and collinear ADS-B antennas in the shops never have more than 4 driven elements.
(Only the ability of the reproducing proves that a plan is well engineered.)
------------------
My words are not carved in stone, and far not all the necessary knowledge is covered by my experience and study - these sentences are for helping beginners with the first steps, not else...
The main rule is: Have fun!
Janos
PS: Sorry for my poor English
Fact:
- For ADS-B reception, a resonant antenna at 1090 MHz must be used.
The higher the frequency at which we scale an antenna, the more accuracy is required. In our case, a deviation of some less than a millimeter can separate success from failure. For complex antennas, these errors not only add up, they can cause exponential deviation.
1. / Wavelength for 1090 MHz
If the inaccuracy given by rounding is allowed, the quarter wavelength is 68 mm. (If we want to be more accurate then 68.8 mm) But! This is only true when radio waves measured in the air. In different materials, radio waves travel different lengths in a given time. The ratio that shows the degree of deviation is the Velocity Factor (VF). This number must be multiplied by the calculated value to obtain the antenna parameter. For copper and aluminum this is 0.95 (95%). For the materials we use, this means 65.3 mm. We can look for the data of the coax cables on the manufacturer's data sheet, where we can also find the VF value for them. (There are antenna parts that often can be made of coaxial cable only. For example: balun --> BALanced-UNbalanced transformer)
Our antennas are always made of a material other than air, this is why we can never use a 68 or 69 mm lenght as a quarter wavelength for cutting!
The formula we use for getting the full wavelenght:
Speed of light / frequency in MHz multiplied by the Velocity Factor (VF)
300 / 1090 x 0.95
--> If you are not interested in details, skip this part and continue reading from the next paragraph.
-- In practice, remembering the above relation, the required conductor length and diameter for a lower frequency give a ratio to which, even taking into account the skin effect -> the Velocity Factor can be as high as 0.98.
In the case of microwaves, the skin effect only works up to a thickness of a few microns on the surface of the conductor, but the ratio between the length and diameter of the conductor is already significantly smaller. As I mentioned, in our case the VF value for copper is 0.95 (95%) which can decrease to about 0.94 at the highest frequencies even if using a thick conductor.
Careful work at higher frequencies is much more important than in other cases. Due to the skin effect, a high-gloss, preferably the perfectly smooth surface is necessary at the highest microwave frequencies. If we have an imperfect surface, parasitic phenomena may appear near the surface. Even really small gaps and scratches can detune the VF and other parameters.
2. / Quarter wave antenna (ground plane, cantenna and the like)
We clarified above that the quarter-wave radiator of the correct size is 65.3 mm (copper wire). The center of the Spider antennas are usually N or F connector - or something similar. The length of the driven element (pin in the middle) must be measured from the point where it emerges from the connector. Try to aim for an inaccuracy of less than one percent.
An important part of the antenna is the additional "pieces of wire" or sheet metal (whatever its shape) placed around the driven element - these wires also must be resonant at the given frequency. Do you remember? - 65.3 mm and 1090 MHz The best is cutting close to the aimed lenght but not over, then using a fine emery or nail file for the last tenths of millimeter. Without caliper, 65.5 mm is ok.
Almost everyone makes mistakes here:
- If the metal shield of the connector is visible above the joints of the wires around, the visible part must also be included in lenght of gound plane elements. The shield seen there also an active part of the antenna! The threaded part of the F extensions are almost always visible, but this is not so bad, at all. (mainly when working without serious measuring devices) The visible part above the radials can also be considered as a temporarily adjustable element so that we can test the direction of change before deciding about cutting (or not) the next tiny part of the wires or the metal plate... Remember? Due to a 1-2% difference, things can go in the trash.
You could say: Mobil antenna owners never cut the roof of the car around the antenna! It is true, but they use antennas with hidden tuning elements in their antennas to tolerate the the differences of the given surface. Mobile antennas always have compromises and they are far from ideal.
- For Cantenna owners: I recommend the fact that from the protruding connector to the open end down below at the bottom of the antenna - "the entire lenght of the can plus the curved bottom from its center" - should be included in the measurement, not just the downward bent part! (Imagine a 1 meter circular plate where only the last millimeter is bent downwards ... Don't you think that only that 1 millimeter counts?
From imagination back to the real world: --> The total electrical length affects the operation.I should mention here that the input impedance of SDR receivers can be of two types. The older ones - still fitting the TV - are 75 ohms, the newer redesigned ones are the 50 ohms (with SMA or BNC connector). We'll get back to this subject soon.
The antenna rods of ground plane are always shown in the images when bent 45 degrees down. Not by accident, but it is not a mandatory rule for all. Amateur radios use 50-ohm input, auxilary devices, and feed lines to avoid signal loss and/or equipment failure. When feed line and antenna have other values than 50 ohm, operators use matching technics. (antenna tuner, balun, unun, choke, etc.) All the components have the same impedance - and where there is a discrepancy, they are made identical to the connected component by tuning or tricky fitting. Sometimes these methods have limitations and losses but in those circumstances they are acceptable and are within the safe value range...
- For owners of a 75-ohm SDR receiver, I recommend using standard 75-ohm coaxial cables - these have better parameters than the same-priced 50-ohm coaxial cables, and last but not least, the 75-ohm dongle fits this kind of cable. There is only one more thing that needs to be changed on the antenna. Bend the ground plane wires horizontally (90 degrees from the driven element) or use a flat, horizontal plate with 130.6 mm diameter instead. (131 is ok, since it is within 1%) Do not forget adding the visible part of the shield (height) of the connector above the plate. Now, you have a matching 75 ohm antenna.
By bending the wires 45 degrees down, the antenna will be 50 ohm. When fully folded down, it is about 32.5 ohm, but in our case it cannot be used.
The radials as ground plane serve and perform several functions. In addition to being a resonant part of the antenna due to its length, it matches the impedance to the cable at the set angle and performs the function of decoupling. (Explains to the antenna that the cable is no longer part of the antenna itself.) ... and also affects the radiation pattern.
Ground plane radials are actually elements converting capacitance to inductance and vica versa. This is already a topic in specialist books and quite boring ...
3. / Franklin collinear antenna - sometimes called vertical centerfed (2, 3 or) 4 half wave
Of the wire antennas, this can even be created without using expensive instruments. Franklin antennas usually have 2 or 4 half-wave radiators. The latter is already a hard nut to crack without at least an antenna-tester or a nano-VNA on the table - but possible to create with a mountain of patience.
The half-wave radiators are 131 mm long, and the phase-matching U-elements are also made of 131 mm wire. Squeezing the "legs" of the U little by little, the frequency decreases but the coupling strengthens. Radiation pattern and gain of the antenna depends also on distance of the neighbouring active elements. So, do not chose a too small distance between U legs! This construction does not allow you to pick the ideal distances (0.7-0.9 WL) between Imax points of halfwave elements for achieving the max gain, thus we have to get the antenna resonant at 1090 MHz with the proper phase delays between radiators, at least.
In the case of half-wavelength elements, the distance between the current-maximum points (the middle of the half-wavelength parts) can start from a minimum distance of 0.5 when the distance between the endpoints is Zero - because we can find a quarter-wave part both downstream and upstream of the center of the elements.
The bigger the distance than 0.5 -> the greater the distance between the endpoints of half-wave elements.
The common theory saying that by doubling the number of half-wave elements we will have an extra 3 dB gain is not always true. Most of the times it is NOT fulfilled. Gain is also a function of the antenna design, the distance from ground surface, and the conductivity of the ground.
You can read from the graph below that a Franklin 4 element antenna with its small endpoint distances - with a bit more than 0.5 value on the x axis - can have 7dBi omnidirectional gain, but it is far from the optimal 8.8 dBi gain.
...graph shows dBd - gain above a single dipole...
Because of the sensitivity of the construction, take care of the radiators being in straight line by creating really precise bendings and fixing them to a common support stick (made of a neutral material like wood or fiber glass). The joints on the supporting stick shall not be fully fixed while you are still working on the antenna. (tune it really carefully since a part of a milimeter also counts)
The U element in the middle of the antenna is the feed point to join the coax cable. Using small "2-pin wire joint with tiny screws" on the legs of the feed point (do not forget sliding it onto the legs before bending the radiators) makes it possible to find the 50 or 75 ohm point without soldering again and again. Be patient, since you have to change the coax position little by little to find the optimal place. By tapping on different points on the stub, tuned frequency slightly changes as well. ...a little trick to make the process more comfortable: The factory made joint of the 2-pin connector must be cut and unnecessary parts removed to keep the ability of tuning the distance of the middle U legs.
For compensating the frequency shifts, all of the U parts should be finely and evenly tuned at the same time. In practice, the two phasing U on the sides have to be symmetrical, at least. So, you have to maintain the distance of the legs on 3 U elements and also the position of your coax on the center one. Any of the mentioned points on the antenna can mess up the final result. Make sure that the path of the coax leaves the antenna perpendicularly - do not lead it in parallel with the radiators and the matching stub (middle U element) before it is farther than a full wavelenght if possible. (from 28-30 cm, the distance is safe) Joining an extra wooden stick to the middle of the support stick already used for the radiators will help. They form a T.
/// I have a nano-VNA but it was still not so simple to match and tune this antenna. At the end, it had 50.23 ohm and 1.07:1 VSWR @1090 MHz, (29.4 dB return loss) ///
(note: PVC also detunes your antenna, avoid the use of it as material of suppport element)
4./ Loss of coaxial cable
There are ways to decrease or to get rid of cable losses. Use low loss coax at the impedance of your receiver and use Low Noise Amplifier (LNA) to compensate the expected loss. Amplifier shall be used at the antenna end, since the already lost or non existing signal can not be amplified at the receiver side.
Use of LNA is more important if you want to use also a filter. These filters are good to keep the unwanted signals overdriving the receiver away - but filters have a so called insertion loss...You can forget cable losses by not using the coax cable at all, or by use of a really short one (pigtail). Not a joke.
These days, excelent USB extension cables are available.With a 10m lenght (30 feet) you do not necessarily have to insert extra power source for your dongle. The extension cable must be a USB2 compatible one. Beware of the ones for keyboards and mice only, because these cables have very low data rate, not like your dongle. (indoor, upstairs or attic) Connecting the antenna directly to the dongle is best. This way, you can forget bothering with coax cables. Cons: USB extension cables have the USB male plug and also an even bigger female end fixed on the cable. This is why you have to chose between drilling a huge hole - or cutting the factory-made cable to use a normal hole through the wall, and solder the previously cut cable back... I do not have to mention that this latest is against the garantie. 5./ CoCo (coaxial collinear) antennas
My opinion strictly... It is a time wasting faulty construction, mainly at 1090 MHz. There are too many cuts of elements for gathering the mistakes and almost impossible to set the necessary parameters even with Vector Network Analyzer. I have never seen a plan of coco antenna where both decoupling and matching were set correctly. There are really few documentation or study about coco antennas. Not by chance. It is really cheap and told to have so high gain, then why the hell we can not buy one in the shops? Let's see...
Intermezzo: Maniacs build them with 12 and 20 elements LOL
My opinion: Above 8 elements the extra gain is almost nothing compared to the problems we may get. Let's say, we can get it work as expected. A 8 or more element collinear has a really sharp plate-like radiation pattern, so we would lose the planes above us, plus any deviation of the antenna from the real vertical would cause even more losses. The round-plate-like pattern would cut into the ground too early - instead of catching planes in the air. The previously mentioned 1% tolerance is not acceptable here, since 1% shorter elements would down-tilt the lowest lobe of the radiating pattern, 3-4 degrees below the horizon. (it is ok if you run a repeater station on the top of a hill and you want to serve the ones in the walley. This down-tilt feature can be set easily at lower frequencies, dealing with longer elements)
So, this antenna tuned to either a bit lower or higher frequency would distort the radiating pattern. With the gain of 10+ elements you can not compensate the possibly distorted radiating pattern, the bad SWR and the mismatch of the antenna. Those who say the opposite tend to forget that the measured bad SWR at the antenna and/or the missing decoupling will always cause more loss at the other end of the coax, and the experienced loss is not linear.
(The values measured at the cable end seem to be more favorable than the real ones, but we cannot read the real properties of the antenna there. The loss of the cable also attenuates the amplitude of the reflected waves, so on a high loss and/or long cable we can see parameters close to perfect even if in reality the antenna is unusable.)
Note: Just pushing the coax pieces together is not antenna making, at all...
Assuming that the antenna is already matched correctly and the common currents of antenna and feedline are decoupled - we still have problems.
The phase shift calculated by the velocity factor of the coax cable is fine, this will force the outer surface of the adjacent antenna elements to radiate - even though they are NOT actually resonant at the calculated frequency due to their physical length. Braid radiates to air, so we should use another VF value here --> but we can not. The measured cable pieces can not be matched for both dielectric materials at the same time.
In summary: Though it can work after a lot of engineering job, but its effectiveness is questionable.
I admit, I also made some of this kind, but when I measured the parameters I would have liked to cry. I sent the final products to the trash.
---------
---------
A really well made, 4 active element antenna will almost always work better at the same site, on the same position. You can achieve greater than 8 dBi gain, at 2° just above the horizon - so keeping the antenna vertically(!) is important, since 1-2 degree of inaccuracy may result in failure.
If you set the antenna to the side of a mast, you can "tune" the gain caracteristics by picking a side-distance from 1/4 to some more than 1/2 wavelenght. At 1/4, it is almost round-like with some loss towards the mast and some more gain in other directions. At half wl, you will get the maximum 9 dBd (~11dBi) gain towards the sides. This is more than enough for an omni antenna.
see: Stacked-phased antennas... You can expect 6dBd (8dBi) of average gain.
----------
By the way, collinear antennas are one of my favourites --> they deserve some attention mainly in vertical use.
...The whole paragraph about CoCo antennas is my personal opinion. You can still love them.
The commercially made, really good antennas are never coco-s, and collinear ADS-B antennas in the shops never have more than 4 driven elements.
(Only the ability of the reproducing proves that a plan is well engineered.)
------------------
My words are not carved in stone, and far not all the necessary knowledge is covered by my experience and study - these sentences are for helping beginners with the first steps, not else...
The main rule is: Have fun!
Janos
PS: Sorry for my poor English
Last edited:
