Antenna tuning.

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Crash one
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Antenna tuning.

Post by Crash one » 28 Jun 2019, 20:37

I’ve built a 120deg half wave dipole for airband TX-RX,
I was told to make each element 23 inches, which I’ve done, within .010”
However it seems from various calculators, and my own, 234 over mid range 127Mhz times 12.
I get 22.11 inches.
I also used 8mm od cupronickel tube cos I read somewhere that it’s better if the tube is 12mm rather than a bit of wire?
How much difference would this make?
I’m no expert on this stuff, in fact completely un informed!

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Re: Antenna tuning.

Post by dt307 » 28 Jun 2019, 21:05

the thicker the element tube vs wire will effect bandwidth due to the skinning effect as rf flows on the outside .
also the above will also have some effect on the resonance of antenna.

use a swr meter to set up the antenna m8 its what they are for.
on hf a few inches makes not much difference but on vhf 2" would be maybe 10 mhz out of resonance
cut it a little longer than a standard antenna calculator and trim to resonance to get the lowest swr
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Re: Antenna tuning.

Post by Tigersaw » 28 Jun 2019, 23:04

You intend to TX on airband?

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Re: Antenna tuning.

Post by Crash one » 28 Jun 2019, 23:43

Thanks for that. You say the thicker tube will affect bandwidth, in what way? I read somewhere that the diameter should be divisions of its length. I worked out somehow (forgot how) that 12mm would be best?
I also discovered that using an swr meter in a hangar with a steel door is not a good idea!
So if I were to remove the antenna and test it outside the aircraft, would the testing/tuning make sense when it is replaced, the aircraft is wood and fabric but it has an engine four metres from the antenna.
Sorry for what may be dumb questions but this antenna is buried low down in the rear rather than the normal sticking up from the roof.

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Re: Antenna tuning.

Post by Crash one » 28 Jun 2019, 23:48

Tigersaw wrote:
28 Jun 2019, 23:04
You intend to TX on airband?
Yes. I have a FRTOL so I’m legal.

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Re: Antenna tuning.

Post by Tigersaw » 29 Jun 2019, 00:03

OK understood, but if I was fitting RTF in a aircraft I wouldn't be making my own antenna. Not sure what SRG would think of that.
Just be aware that a low SWR does not indicate correct resonance, that only indicates the best match to the radio, which fortunately in the case of a dipole in free space is pretty much the same, but as your experience has shown can give wild readings, including false positives if you get it the right kind of wrong if that makes sense.

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Re: Antenna tuning.

Post by Crash one » 29 Jun 2019, 09:33

I think I’ll remove it, cut it to 22 1/8” each end replace it and check it as best I can with the swr meter. Engine running and in the middle of the field out of everyone’s way.
As for using a home cooked antenna rather than a commercially available generic item.
The whip it has/had uses four lengths of thin braided wire with loose ends festooned around the cockpit, looks untidy, never worked very well with the old 25Khz set nor a new handheld 8.33 set. So with this new, panel fit 6watt setup I’m hoping for reliable clarity both ways, usually no further than 50miles. I assumed that the designer of this thing knew what he was talking about, he uses one on a microlight, made from coat hanger wire. I thought I could do the same or better.
I was just curious why 23” rather than 22.11” (127Mhz mid range)
If this swr meter I’ve got is going to give me spurious readings it’s hardly worth the effort because I wouldn’t know whether to trust it or not.

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Re: Antenna tuning.

Post by Tigersaw » 29 Jun 2019, 11:32

Frankly I doubt the .8 inch will make much difference - formula are fine for free space calculations, but in the real world you suck it and see. As mentioned by another poster, make it too big and trim some off as / if required.
Good test might be tune to a nearby ATIS and try different locations, orientations etc

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Re: Antenna tuning.

Post by Crash one » 30 Jun 2019, 00:49

Tigersaw wrote:
29 Jun 2019, 11:32
Frankly I doubt the .8 inch will make much difference - formula are fine for free space calculations, but in the real world you suck it and see. As mentioned by another poster, make it too big and trim some off as / if required.
Good test might be tune to a nearby ATIS and try different locations, orientations etc

Different locations, orientations etc???
This is an airborne set up, have we got a couple of crossed wires here?
Anyone in Fife area tomorrow Sunday there is, weather dependent, an informal fly in at Kingsmuir air strip. For miles north of Anstruther and half mile west on the B 940.
Listen out and or come and visit 135.480 (safetycom).
There were a number of aircraft there today Saturday in the circuit till the thunderstorm hit.

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Re: Antenna tuning.

Post by Tigersaw » 30 Jun 2019, 08:50

I meant in the airframe. Assuming you have the space to move it about, try different positioning. Airband antenna are normally vertically polarised.

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Re: Antenna tuning.

Post by Crash one » 30 Jun 2019, 10:12

Tigersaw wrote:
30 Jun 2019, 08:50
I meant in the airframe. Assuming you have the space to move it about, try different positioning. Airband antenna are normally vertically polarised.
Ok, Sorry. It is mounted vertical facing backwards with about 8” projecting out the top.
No where else it could go. The plastic plate/mounting is bolted to the only place it can on the rear bulkhead behind the parcel shelf.
3.6 metres of RG 223 double shield coax.
I don’t actually have a problem. Just curious as to how critical it is for the length to be perfect. Seems that it’s not vital so I’ll probably cut a bit off it just to be pedantic, or leave it.

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Re: Antenna tuning.

Post by Crash one » 02 Jul 2019, 16:07

One other question. The plastic mounting plate is attached to the bulkhead vertically, which determines where it passes through the roof. If I want to move it further down I either have to drill another hole in the roof or bend the top element from 30 deg rearwards to vertical for a few inches then rearwards again back to 30deg. I don’t want to drill any more holes.
Will this upset the performance in any way?

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Re: Antenna tuning.

Post by Tigersaw » 02 Jul 2019, 18:07

I'm going to guess any performance detriment will be marginal. You are not changing the overall length, you are not bending the element such that it turns back on itself. Its not ideal but needs must. Just to check, you are orientating it so that the element connected to co-ax centre is the one pointing upwards?

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Re: Antenna tuning.

Post by Crash one » 03 Jul 2019, 11:39

Yes the centre core is pointing upwards.
Thanks for that, looks like this thing will work ok.
Thanks for your help.

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