Hand held radio.

[Trevor Harvey]

Thank you.
Further silly question. Regarding aerial material, is it a good idea or not to use 4mm brass tubing rather than a solid wire for the dipole units?
Also on the subject I note that my existing external aerial coax is the single strand solid core wire type, I'm now under the impression this is not RG58 ?

[dmcnicholl]

Regarding aerial material...

The dipole antenna link given by Ian Law near the start of this tread mentions 2mm wire, brass rod or coat hangers! I have no idea if a tube would be a good idea or not but for preference I'd stick with wire. And the only advantage I can imagine for using brass is the ease of making a solder connection.

Google seems to show that RG58 can be single core or twisted, so in itself not definitive. For a mobile installation though I'd go with twisted flexible rather than solid single.

I have also been told, and I have read, that the ideal length of the coax should be a multiple of the half wave length (based on a centre frequency of 121mHz) so 31" 62" 93" etc. and that while the routing of the cable from antenna to radio can snake about somewhat it should never be allowed to turn back upon itself.

[John Clarke]

I finally managed to test my FTA-750 in the air today. Readability 5 from Norwich radar at a distance of 22 miles from 2000 feet using the standard Yaesu power supply.

As I previously mentioned, the audio quality from the Yaesu is much better than my old Icom. I do, however, appear to be getting more interference from my electric variable pitch prop when changing pitch though this isn't really much of a problem as I only change pitch from fine to coarse and back again once during a flight.

With regard to the MOD7 form, I guess it is a bit OTT but that's not exactly new in aviation. I still regard my radio as a permanent installation and I'll submit a form next week. At least it doesn't cost money, which makes a change!

Edited to add that the MOD7 form requires 14 signatures from my inspector. Seriously, who makes a form that requires 14 Signatures from the same person?

[tnowak]

There are different versions of RG58 coax cable. RG58C/U has the stranded inner conductor.
Always best to use stranded in aircraft but solid is okay of your cable isn't going to be continuously "flexed".
The is a UK company which will make up coax cables for you if you don't have the special crimp tools available: https://www.gigatronix.co.uk/cabulator/ ... cable-type

Tony

[Ian Melville]

The diameter of antenna elements has an effect on the bandwidth of an antenna. As the Airband comms range is only 19Mhz end to end (118-137) there is not a lot of gain in using a tube. In aircraft I would only use rods, especially externally when the drag is a factor. Though for a ground station I may use a tube.

The ideal length of a coax can inded be optimised by using certain multiples. Not quite as simple as a multiple of the wavelength. Unless you have the kit to test the installation, don't worry about it as you are more likely to get it wrong anyway just using calculations.

RG58 is a solid centre conductor, RG58C/U is multistrand. You want multistrand in an aircraft.
RG400 is the better cable and has less fire risk than the PVC of RG58C/U. For the short lengths, we use the performance difference is neither here nor there. TL3.03 states

Only aviation quality wiring and terminals should be used. PVC-insulated wiring should not be used.

If you do use RG400, make sure you use the correct fittings.
Note: RG58 and RG400 are just specifications. There is a variation in quality from different manufactures and is worth paying a bit more.

Whatever you use make sure your antenna is tuned. I tested an installed Comms antenna the other day and found it had a centre frequency in the Nav band. It is sold as an aircraft Nav/Comm antenna, but the installer must have just plonked it in and not bothered to tune it. It was sufficiently far out to risk damaging the transmitter, never mind degrading the performance of the radio!

[Trevor Harvey]

Thank you Ian.
This is getting more confusing with each reply.
The idea here is to build the dipole aerial described in the link from Ian Law.
Fitted inside a wood and fabric aircraft, so tubing would not be a problem ref: drag.
I read this article ex Wikipedia.

Instead, it is often desired to have an antenna whose impedance does not vary so greatly over a certain bandwidth. It turns out that the amount of reactance seen at the terminals of a resonant antenna when the frequency is shifted, say, by 5%, depends very much on the diameter of the conductor used. A long thin wire used as a half-wave dipole (or quarter wave monopole) will have a reactance significantly greater than the resistive impedance it has at resonance, leading to a poor match and generally unacceptable performance. Making the element using a tube of a diameter perhaps 1/50 of its length, however, results in a reactance at this altered frequency which is not so great, and a much less serious mismatch and effect on the antenna's net performance. Thus rather thick tubes are often used for the elements; these also have reduced parasitic resistance (loss).

Not being a radio person by any means, this meant little to me other than to indicate that using tubes may be an advantage. However 1/50 of its length amounts to 12mm or .472 inches which seems a bit on the large side. Though that last sentence seems to justify it.
I could no doubt hand this job over to an expert and solve the problem but being a devout tinkerer myself it seems the construction of said dipole to the dimensions stated would be quite simple to me with my well equipped workshop. On the other hand I don't like working in the dark with a risk of frying something vital.
Frinstance how does one tune an aerial? I have no electronic measuring equipment, would building it to the drawing dimensions be enough? If I were to use, say, 6mm cupronickel tubing used on vehicle brake pipes, am I going to screw everything up?
Apologies for my ignorance of the subject.
My current external aerial has a ground plane of 4 lengths off braided "earth" type wire pinned round the cabin roof and a rod aerial on the roof, solid core coax of dubious quality.

[mikehallam]

Just to further fox you,

My wooden Jodel had its 1/4 wave aerial hanging entirely inside the fuselage, not far behind the cabin, rather than having it stick up into the wind. It didn't seem to care, had the similar foil ground plane stuck inside the roof and gave the a/c even cleaner lines.

mikehallam.

[Ian Melville]

Thank you Ian.
This is getting more confusing with each reply.

What you quoted was a long winded version of what I said. If you want to dig into signal propagation be prepared to do a shed load of maths, and then still find out that theory does not match practice.

Frinstance how does one tune an aerial? I have no electronic measuring equipment, would building it to the drawing dimensions be enough? If I were to use, say, 6mm cupronickel tubing used on vehicle brake pipes,

Building from a drawing you could be in the right ballpark, but not on the sweet spot, but you would never know if the ball was off the pitch, or not.
At the absolute minimum, you need a Standing Wave Ratio(SWR) Meter. You start with the elements over-length and test with the SWR meter. Then trim small bits off until the SWR reading is at it's lowest. If you go too far the SWR will start to go up again, at which point you curse and try an stick a bit back on or make new elements of the correct length, which of course you recorded. It's a slow job and requires you to transmit at a centre frequency. You then need to check max and min frequencies you wish to use and check these are still within tolerance.
I use one of these, https://www.hamradio.co.uk/accessories- ... pd-858.php, but have other kit available. BTW an old CB one will not do.

I have my hands full with 'Stuff' at the moment, but I should be able to graphically deomstrate the difference between 12mm copper tube and 4mm wire.

[Ian Melville]

BTW the airband range is 119-137Mhz, so if you tune an antenna a 127.5Mhz, each end of the range is only 1% off the centre frequency.

Edit: I deleted this post, but it seems the forum had other ideas. The correct figure is 10%, not 1% as I originally wrote . I will run some tests and see just what impact this has to the bandwidth.

[Trevor Harvey]

Thank you Ian.
I think I'll give it a go at the drawing dims and see how it performs. Or get someone to tune/check it first.