Co Phasing Antennas


#1

I have been making my own Yagis which are working out to about 250 miles but I would like to experiment with co-phasing 2 or more yagis and wonder if anyone has details on how to do this?


#2

Dont have details and dont know much either, but know this much that a “Phasing Harness” is required to couple two antennas to one receiver. Google for Antenna Phasing Harness.


#3

The phasing basically requires two cables exactly identical in length and same material.
Example: since the wavelength of the 1090MHz signal it is 275.2mm, a difference of 1/2 of that (187mm) will put the signals in opposition of phase, canceling them.


#4

@blakemulder
It is slightly more complicated than just connecting two cables of the same length in parallel as if you do that with 50 ohm antennas and coax you will have 25ohms at the common feed point of the antennas (two 50ohm systems in parallel).
In order to get a 50 ohm system with 2 antennas you have use 2 lengths of 75 ohm coax cut to an odd multiple of 1/4 wavelength to connect to each of the 50ohm antennas. This will transform the 50 ohms of each of the antennas to 100 ohms. You then join the 2 coaxes together to get back to 50ohms.
Some more information here:


#5

I was going to suggest to use a combiner. https://www.instockwireless.com/power_divider_pd1120_fab.htm
Much easier, especially if each antenna has a preamp, you might not want to tie them directly with cables.
The trick with cables tied together is good for TX.


#6

Yes, that will work too. :slightly_smiling_face:


#7

.

.
Splitter/Combiner have attenuation as follows:

  • component #1
    Power split into two halves = 3 dB
    (not applicable to combiner, applicable to splitter only)
    PLUS
  • component #2
    Insertion loss = 0.3 dB ~ 4 dB, depending on model and make

Other parameters are as follows (values depend on model and make)

  • isolation between antennas = 10 dB ~ 30 dB
  • amplitude balance = 0.2 dB ~ 1 dB
  • phase balance = 2° ~ 15°

.


#9

Something like the mini-circuits zesc-2-11 has somewhat better amplitude and phase imbalance specifications at 0.05 dB & 0.04 degrees respectively at 1100MHz


They are however a bit more expensive than a few lengths of coax.
https://www.minicircuits.com/WebStore/dashboard.html?model=ZESC-2-11%2B
Bargains can be found from time to time however on the used market.


#10

I dont think the phase and amplitude parameters are critical for combining two Yagis for ADSB… Phase and amplitude imbalance will be there any wsy due to difference in cale lengths and different locations of antennas. Not worth spending a lot of money to improve these parameters.

A moderately priced one, with moderate attenuation and reasonable isolation between antennas should be ok.


#11

Sure. But those coax lengths can be critically off, especially when their propagation speed is unknown exactly.
As a “fun” note, the tuned cable lengths were used extensively in the TV - to change over from 300 ohm symmetrical to 75 ohm asymmetrical. Sometimes a good balun was better at that because of imprecision in cables PS.

I think you are perfectly right! The planes are punctual sources of signal, and they will travel from one lob to the other. Maybe at the intersection of the lobes it will matter the phasing, but if the lobes are spaced at 90 degrees… phasing won’t matter, the plane will be in only one antenna lobe at a time.

What I don’t understand is how a directional Yagi, vertical polarized, is supposed to improve the signal. The directive pattern will be concentrated in one small vertical slice/sector, while the horizontal selectivity remains unchanged from lambda/4 antenna. This is not that great.


#12

The ADS-B signal is vertically polarized. A polarization misalignment of 45 degrees results in a 3 dB loss. A misalignment of 90 degrees, up to 20 dB loss.

So the Yagi vertically polarized does not improve the signal because of the ‘correct’ polarization alignment, it simply avoids the mismatched polarization loss. What improves the signal is the Yagi’s gain compared to an omnidirectional (isotropic) antenna.

In our case, ADS-B, we would be dealing with 90 degrees only (up to 20 dB loss). Other degree variations are common with satellite dish feed alignments.

A simple analogy for mismatched polarization would be trying to insert a letter vertically into a mailbox with a horizontal slot.:grin:

Anybody tried an antenna with circular polarization to see the difference, if any?


#13

:rofl: :laughing: :grin: :+1:


#14

I know that mr. Ovious :slight_smile: But that gain comes from “restricting” the angle in the plane that dipoles are sitting. So a vertical polarized Yagi will have a (very) narrow lobe (angle) on that vertical. Not what you want when you are aiming at planes that can be at any elevation - virtually at a 90 degree angle between planes at horizon and planes above head.


#15

I assumed the original poster @blakemulder was intending to stack or bay (or possibly both stack and bay) multiple Yagi antennas so they appear to be a single higher gain antenna. However, if the antennas are mounted in different locations or pointed in different directions then I agree the phase and amplitude parameters would not be so important.
I just use an omnidirectional antenna myself for ADSB.


#16

Might be useful though if you were looking at ground movements at an airfield on limit range.


#17

Neah. The range, on straight line of sight, to see something on ground is only a few miles (depending of antenna elevation). You don’t need Yagi gain for that, you just need clear space between.


#18

If by that you mean reducing the beamwidth by increasing the number of reflectors and directors, yes, otherwise you lost me.

Now you are talking take off angle. If you want to optimize for distance, and this why one would consider a Yagi, low take off angle is what you need.

Unless one has special interests, or live far away from the established air lanes, there is no reason to use a Yagi or any other directional antenna.


#19

But the signal may be attenuated and drowns if you have lots of planes.

So in that case a directional antenna can help to get the maximum number of messages.

Otherwise they could be blocked by stronger signals from other directions.


#20

Exactly. That’s what I meant. If you wanted to receive weak ground messages at the limit of your ground range and exclude most of the airborne transmissions, then a narrow beam width directional antenna such as a Yagi would be the way to do it.


#21

The signals from that range will be strong. We are talking about something like 6 miles here.
http://www.calculatoredge.com/electronics/lineofsight.htm
LOS_6