Franklin antenna for those who have a 75 ohm receiver!

Janos Konya

New Member
--> Feeders with 50 ohm system will also find useful ideas...

I recently made a Franklin antenna model whose most parameters can be changed.
Franklin.jpg

As I played with this, I realized that unreasonably many people were disappointed with this antenna, they had failed in construction. Now those with a 75-ohm system can breathe again. There is hope and simple solution to get rid of most problems!
...In addition, 75-ohm ADS-B antennas are not available in stores...

In fact, I could never see a photo about this antenna on flight feeder forums with the necessary 4:1 or 1:1 balun. No wonder they don’t work or just provide unacceptably bad reception. (comparison of a Franklin and a GP antenna is not even fair but I often read reports about a similar performance.)

Let's change this sad situation...

The most sensitive point of this antenna, in terms of construction, is the central bent element - the impedance adapter. If it is not made of the correct length of wire or you do not find the correct impedance points - the antenna will not work.

... What if we didn’t even use this component? The impedance at the end point of the half-wave antenna element is extremely high - thousands of ohms. This is too difficult to handle. ... it is better to use well-proven methods.
The impedance of half-wave dipole antennas is already very close to 75 ohms. (about 73 ohms and made of 2 quarter wl segments)
Why would we complicate our lives with a different solution?
Let's see.

It will be simple:
- forget the middle U-shape and cut the connected radiators in half
modifications.jpg


- connect the middle conductor of your 75-ohm coaxial cable and the shield braid directly to the endpoints (feed point) Do not use long wires!

There is only one step left. We have to prevent antenna currents from also using the outer conductor of the coax as an antenna component. It is the key movement of the construction.

- Cut a quarter wavelength long part from any remaining piece of coaxial cable -> a little longer one so that the middle conductor can protrude 1-1 cm. Now we are not using the Velocity Factor given by the manufacturer, but the ratio to be used in copper wire. (0.95 so 95%)
Don't even calculate, I'll tell you...
- For 1090 MHz keep the braid intact at a length of 65.3 mm and close it short to the middle conductor at the ends. If you did it right, now you hold a metal rod of the same cross-section as the original coaxial cable, with a spike at the ends. We need it now as a large-area electrical conductor, not as a coax.
- Bend one of the pins at the base by 90 degrees and fit the cable piece - parallel to the cable end attached to the antenna. Attach the straight pin to the center conductor of the coaxial cable (or to the point where it connects to the antenna). The other pin should point to the antenna coaxial cable. Where it touches, carefully remove the plastic insulation in a small area to gain access to the braid. Solder the bent pin there or hide it between the insulator and the braid. The two adjacent coaxies are parallel and have a 2 mm air gap between them.
The reason why we used the VF 0.95 is this air gap and the copper surface of the modified coax.

DC and RF currents both tend to choose the easier path. In our case it will be the braid instead the center conductor where just a smaller part will go through. Thanks to the connections and the resonant quarter wave lenght, we will have mutually neutralizing electric fields between the parallel surfaces of coax parts. Common currents of antenna and coax braid can not flow on the tuned frequency, so antenna elements will behave like an extra metalic object (coax to the receiver) would not be connected there...

Now, the antenna will surely fit the cable well, and the latter will not distort the radiation pattern of the antenna. Not to mention the fact that you just made successfully a 1:1 balanced-unbalanced transformer (BALUN)


- This kind of decoupling method with this 1:1 balun works well also for others with the central impedance tuner U element, either 50 or 75 ohms. This U section is for setting the proper impedance only, but you have to deal with the "common currents" as well. (it is decoupling)
In radio engineering, this 1:1 balun is called: "The Pawsey stub" or "EMI Stub Symmetrising Device". Technically they are identical if there is an air gap in both solutions...
If you stay with the U stub and pick a bit higher impedance, - 200 or 300 ohm depending on your 50 or 75 ohm coax cable (tap points are about at the lower third of the U) - you have to use a 4:1 balun. I have already written about it in another post.

The 1:1 balun shall be chosen for decoupling when you already have the same impedance on the antenna and the coaxial cable.
The 4:1 balun is for matching and decoupling at the same time.
(the ratio indicates the impedance transformation if there is)


Using this sizing in 75 ohm and decoupling method with a balun in both 50 and 75 ohm systems, the functionality of the antenna is almost certain.

In the 75 ohm version, there are only 3 half-wave antenna parts instead of the original 4.
In return, we got rid of most problems. Keep in mind that the much-loved FlightAware 66cm antenna has a gain of 5.5 dBi.
The Franklin version we just made is 7-7.2 dBi - so we still have no reason to complain. :) Of course, the choice is yours but without antenna tester or VNA - I suggest the half wave dipol middle part for 75 ohm systems...
If you can not see a plane, not the 7dB gain is the problem but the shape of Earth or the position of the antenna, maybe the objects and buildings around it. An unnecessarily high gain antenna can cause problems as well...
Anyway, a well made lower gain antenna is always better to use than having a non working one with higher gain (according to its specs only).


For sure, I will also specify the other parameters:
- Radiators at the ends of the antenna 130.7 mm (including the curved part at the bendings)
- the quarter-wave elements forming the middle half-wave dipole are 65.3 (65.5) mm long
- Between the lowest point of the two phase-delay U-elements and the nearest point of the radiators (measured not from the center of the wire) 66 mm. The distance between the stems of the U is 6 mm (between surfaces), which you can later compress slightly or open a bit more as needed, but the stems should remain parallel. In fact, it is a matter of straightening or further bending the already bent part at the bottom. (do not forget keeping the stems parallel) This allows you to fine-tune the length of U , ...just parts of a millimeter if necessary.
- You can attach the radiators to a wooden stick so that they do not move.
- It is best if the coaxial cable is routed at a steep angle from the antenna elements, including the phase delay U sections.
- Use a bare copper wire to make the antenna.

+1 tip for building antennas with more than one driven element or radiating parts:
Do not finish your antenna in a single workflow and hope that it will surely work. If possible, build the antenna's logical sections one by one. In case a Franklin antenna, create the matching section and the connected radiators first. When the "module" works fine, you have the perfect measured lenghts for the next module. Just copy the parameters and work with them, since the used materials and their properties are not always the same - so you can not trust your calculator or a recipe alone. Otherwise, you will get a W-like zigzag on your tester, and you will not be able to determine which element is not resonant at the designed frequency. Also keep in mind that getting an acceptable SWR sometimes means nothing about the quality of your antenna. My 50 ohm dummy load is "really wide-band antenna" with 1:1 VSWR everywhere, but it will never get a signal from outside... :)
So, check the units one by one and follow the well proven practices for the whole project.

...back to the Franklin antenna...
If you are satisfied with the result, seal the cut surfaces of the coax with hot glue for outdoor use. (better than two-component resins as they detune the antenna)
You can still use the wooden wand to mount the radiators. Apply several times a mixture of white spirit (or alcohol) and oil, using a piece of cotton wool then allow to dry. Then make it water repellent by waxing. In case of rain, the antenna will not become temporarily unusable or the chances will be lower.

Good luck with the construction! (maybe just a conversion)

Janos
 
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