Ground plane for transponder antenna?

[Soleair]

I am getting a Rami AV22 transponder antenna fitted to the underside of a Jabiru. The fitting instructions make no mention of a ground plane.

 

It is/was my understanding that a ground plane is required for this, of at least 1/4 wavelength dimension.

 

But there seem to be differing views on the size & shape of the optimum groundplane. I have found those who suggest round is not ideal; round 305mm dia; square of side 120mm; copper tape in + format 68.8mm leg, fit inside fuse; fit outside fuse, etc.

 

Does anyone have this aerial fitted, with a groundplane, who'd care to comment?

 

Thanks.

 

Bruce

 

 

[jetjr]

Jabiru have install document on it http://sydney.edu.au/engineering/aeromech/AERO1400/Jabiru_Construction/Manuals/J160/Construction%20Manual/INSTRUMENT%20PANEL/Fit%20Transponder%20Antenna.PDF

 

I made mine from thin alloy sheet

 

 

[Soleair]

Thanks jetr, that's very useful. But can you tell me the diameter of the ally plate? I can't find it on the attachment.

 

Cheers

 

Bruce

 

 

[techie49]

Hi Bruce,

 

I wouldn't get hung up on the size of the ground plane. As you correctly state, anything larger than a quarter wave at your operating frequency will be ok. As big as is convenient would be my measure. It only serves to allow the antenna to 'match' with the transponder and also helps a quarter wave antenna avoid having only high angle radiation. To get maximum range, your antenna needs to radiate at a lower angle. The learned books will show in polar patterns what I mean. Make sure you follow any guidelines for cutting the antenna to resonance at the transponder transmit frequency as I doubt you'll be able to measure the SWR of the system. Hope this helps a bit.

 

Paul

 

P.S If you want an absolute measurement, I'd take the highest, (transmit) frequency of 1090 MHz, the wave length at that frequency being 27.5 cm, so a quarter wave would be 6.9 cm. I'd go for about 10 cm for ease of measuring and cutting. Think about mounting an antenna on a normal car. The ground plane is a little bigger than a quarter wave, certainly at VHF and UHF !

 

 

[Soleair]

Thanks techie. Very useful info.

 

 

[skippydiesel]

Hi Paul,

 

Tailors & dress makers use CM (God knows why) all others should use MM or M. A tad pedantic I know but it is easy to confuse people using different language to describe what should be a simple measurment.

 

For my transponder antenna (in composite air-frame) it was easy to use a 100 x 100 mm square of scrap aluminium as my ground plain. It seems to work very well.

 

Perhaps perversely I used a cross shape for my radio antenna, mounted on underside of aircraft. I found this to fit my aircraft better than a square or round sheet. This ground plain is combination of scrap aluminium sheet, in a narrow strip, running along the aircraft "belly" and soft aluminium builders flashing running at 90 degrees and curving a little way up the internal sides of the aircraft. Again I get very good reception and compliments on transmission clarity. Sorry cant remember dimensions but can get them for anyone interested.

 

 

[techie49]

Hey Skip,

 

I was brought up on feet and inches but since some look at you blankly when you mention those units I thought I ought to put measurements in terms some would understand!

 

BTW If you do want to be pedantic, it's a ground 'plane' .........(!)

 

Paul

 

 

[IBob]

Hey Skip,I was brought up on feet and inches but since some look at you blankly when you mention those units I thought I ought to put measurements in terms some would understand!

BTW If you do want to be pedantic, it's a ground 'plane' .........(!)

 

Paul

.......and it's m and mm, not M or MM.........)

 

 

[skippydiesel]

Fella's!! - M & MM to emphasise m & mm. Note I used it correctly in the sentence following.

 

My craven apologies for the plain & plane

 

BUT

 

Did you like my ground plane solutions??

 

 

[IBob]

Fella's!! - M & MM to emphasise m & mm. Note I used it correctly in the sentence following.

My craven apologies for the plain & plane

 

BUT

 

Did you like my ground plane solutions??

We just pushin' your buttons...........)

 

Most excellent description of ground plane!

 

 

[techie49]

Buttons definitely pushed I'd say...........!

 

Skips reply illustrates what I said about not stressing over the size of a ground PLANE (!). Whatever works for you is ok. Decent wiring and antenna connections are as important.

 

Paul

 

 

[IBob]

Thanks all the knowledgeable people her.

 

As a newbie builder not yet up to these bits, I'm reading and absorbing.

 

This is the gold for me on this site.

 

 

[mnewbery]

 

 

 

[techie49]

Well a little narrative would be good !

 

The pics seem to show a comms antenna inside a structure with an enlarged washer beneath it. If it's a normal VHF antenna, I'd be interested to know what sort of a match was achieved in s.w.r terms. I'm fairly sure you could transmit and receive using it, but the performance could probably be improved. Do I pass?

 

Paul

 

 

[techie49]

On closer inspection ( first read on my phone) I can now see a rather better set up having completely missed the copper strips in the pic ! ! !

 

So I didn't pass first time !

 

One point however. What is the material used for the 'washer'? I'd have gone for tinplate and soldered the copper strips to the washer. If it's aly, you are setting up for future problems with dissimilar metals in contact.

 

Paul

 

 

[mnewbery]

Is it a Fisher Flying Products R-80 with an ELT stuffed in its guts (that would be the yellow box in the linked image) or a E-7 wedgetail you want to build?

 

 

[skippydiesel]

Terrific workmanship - bit drafty though.