1/2 wave mopole

#1
Hi All,

I wanted a low gain antenna and had a think for an easy design above the basic 1/4 wave antenna's.

I came up with this, a 1/2 wave mopole type.

For a test concept I went through several variations before this so far final has proven to work well to date.

List of materials, 1 BNC panel mount socket or a connector of your choice and 290mm of RG-59 coax cable "solid centre conductor" plus solder and iron.
Needle nose pliers is an advantage for some bending of the wire.

Construction, first remove the outer jacket and shielding from the coax so you are left with the center conductor and dielectric,
With the conductor cut no more than 30mm of centre conductor and dielectric off and then strip off 5mm of dielectric to expose centre conductor
and solder to the BNC centre pin, NOTE make sure it is a clean solder and no little solder tabs hang out if so trim tabs back.
Then measure and trim length to 19mm from base of ground point around centre pin to end of conductor.

Next with the remaining centre conductor remove the dielectric from the conductor.
With that done measure from one end 10mm then make a 90 degree right angle bend,
from that bend measure out 69mm then make a 90 degree left angle bend.

Now with the aid of some RG-59 coax place the first bend against the coax and coil the 69mm length around the coax with 1mm gap between each loop till you loop up to the next bend.
Now you should have 3 loops to form a coil around the cable, gently remove off and place this over the centre conductor on the BNC socket, the coil needs to be centred and at the lower end, use some outer jacket from the coax make sure you remove the shielding and slip over centre conductor and this will keep coil centred the remaining wire at an even distance from the conductor.

Now with the 10mm start end, trim down so the start of the coil is about 2mm above the base and solder that to the ground of the BNC panel socket.
Check to make sure every thing looks good, next with the remaining wire above the coil measure this length to 137mm and cut off excess wire and with a plastic cable tie / zip tie secure the whip section "not to tight" to the centre conductor to keep the gap even between the centre conductor and the radiating element.

If you done things right it should yield some good results.

Testing was done at 3 meters above ground level, I'm surrounded by tall trees 15 to 25 meters high with high hills to my North, East and South and got results slightly better than a monopole 1/4 wave that resided on the test mast before hand.

As I have no real testing equipment I cannot say if the antenna is at peak performance or accurately tuned it was alot of guess work and crude calculations based loosely of another antenna for a much much lower frequency.

Gain is at a guess around 2 to 3 Dbd and range rings are at 50 nm.

Gary.
 

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#3
The other thing I didn't write is on my test mast is the very long cable run causing a sizable loss 11 db and no pre amps are used, If I placed the receiver at the aerial or used a quality coax cable for the length used the results would be better. But it is a way for me to compare aerials with a constant known.
 
#4
I mounted a second 1/2 wave mopole antenna at 10 meters above ground level, the attacked picture the outer grey is my 1/4 wave monopole ant at 18 meters, the red is the 1/2 wave mopole (20 hours up time) at 10 meters and the inner grey is my 1st 1/2/wave mopole at 3 meters, Range rings at 50 nm.
Gary.
 

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ab cd

Senior Member
#5
Your design seems promising.
Since range depends on many factors, a realistic & meaningful comparison would be when all 3 mono-poles have:
1) same height
2) same location (horizontal distance about 0.5 meter to 1 meter)
3) same type & length of coax to their receivers
 
#6
That's my plan, when conditions improve I will lower down the tower so I can place both existing and the 1/2 wave mopole aerial's at the same height and then give some time to get some data and see how it really stacks up.
 
#7
After a month and a bit, I had some time to get some rough non scientific data, Both 1/2 wave mopole and 1/4 wave located 12 meters above ground level and 1 meter apart horizontally and did a rough comparison, I had a very slight range improvement over the 1/4 wave and about 6% improvement in messages received, the only real difference I noticed with the 1/2 wave mopole is the receiver didn't get much interference from the nearby broadband data service at 1085 mhz where the broader frequency coverage of the 1/4 wave the receiver did suffer, Posted picture 1/4 wave is in red, 1/2 wave mopole is in grey.
 

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ab cd

Senior Member
#8
Thanks for sharing very useful results, particularly information about reduced interference from 1085 MHz data service.
 
#9
Been meaning to get this up, using sisex on RTL1090 the image on the left is the 1/4 wave aerial and the image on the right is the 1/2 wave mopole aerial on each screen shot 1st image is with no interference as the adjacent channel data system is in between transmissions, 2nd & 3rd image is when the data system is transmitting you can see slight interference on the 1/2 wave mopole but compared to the 1/4 wave it's nothing. found it's licence it's a 100 watt site bugger.
The 1/2 wave mopole aerial may not be perfect, but in my situation it's better than what I was getting before I made it.
 

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