A question on aerial length

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Dodgy Geezer
Posts: 62
Joined: 17 Jun 2018, 09:54

A question on aerial length

Post by Dodgy Geezer »

For many hand-held transmitters the aerial is telescopic, sliding out of the Tx box when operational.

When you slide it out of a metal Tx box some of the aerial remains inside the box - supported by the insulating mount. Perhaps an inch or two inches remains inside this metal cage.

Does the aerial left inside the metal contribute towards radiating efficiency? Or is the only functioning part of the aerial that part which is free of the metal box? My guess is that the resonant length of the aerial includes the parts left inside the cage, but that the actual radiated energy only comes from that portion which is outside. So if you were calculating a precise aerial length you should include the bits which never come outside of the Tx? Does anyone agree....?
Pchristy
Posts: 413
Joined: 16 Feb 2018, 13:57
Location: South Devon, UK

Re: A question on aerial length

Post by Pchristy »

The precise aerial length is generally unimportant provided the transmitter is properly tuned to it.

On 27/35 MHz, a telescopic aerial will be less than 1/4 wavelength long. This will make it appear capacitive in nature, and it will require an inductor somewhere to tune out the capacitance. In days of yore, this inductor might have been half-way up the aerial (centre-loaded), but most put the inductor at the base somewhere, making the aerials cheaper to source. It also made them a bit less critical length-wise!

Base loading coils are generally tuneable to allow matching to an individual aerial. At 35 MHz, the inductance required is around 1uH for a standard telescopic aerial. In addition, there will be a matching network - the equivalent of a gearbox - to match the radiation resistance of the aerial (typically between 30 and 50 ohms) to the output transistor. On 27/35 MHz, you typically want around 1 watt out to generate the legal 100mW Effective Radiated Power (ERP). The supply voltage squared divided by the required power output will give the impedance that is required to deliver that output.

So for a 8-cell battery pack (9.8V, but lets call it 10 for simplicity!) the output transistor needs to see 10^2/1 (10 squared divided by 1) which equals 100 Ohms. So the matching network need to transform 100 ohms down to between 30 and 50 ohms. (This is why they are made tuneable, as it is difficult to calculate the precise impedance of a hand-held box with a telescopic aerial!)

The matching network will usually takes the form of a resonant circuit which also provides filtering for unwanted harmonics of the output.

In practice, an inch or two of difference (maybe even more!) in the length of a base loaded aerial will not make a big difference to the output. On a centre-loaded aerial, the length of the top section is quite critical, being shorter and doing most of the radiating.

I do not like centre-loaded aerials! They are more expensive, only work on the design frequency, are more easily damaged and only really radiate from the top section. For our purposes, aerials are a bit like loud-speakers - the bigger the better, up to a point (quarter wavelength) beyond which things get impractical.

If you want to know the impedance of a whip (telescopic) aerial, there is a good calculator here: https://daycounter.com/Calculators/Whip ... ator.phtml

The diameter should be the average diameter for a telescopic aerial (measure half-way along). Have a play with different lengths and you'll see that for small differences, not much changes!

:)

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Pete
Dodgy Geezer
Posts: 62
Joined: 17 Jun 2018, 09:54

Re: A question on aerial length

Post by Dodgy Geezer »

Thank you for a most comprehensive post on aerial length, which covered a lot of ground! Perhaps, if I explain what prompted my query, you might be able to provide a more detailed comment on my problem?

I am looking at refurbishing old single-channel radios. Most parts are easily replaceable, but the original aerials are always a problem, since telescopic aerials are a moving part and either get broken or worn. They can be replaced, but rarely with an item of exactly the same length. Interestingly, I note that the original aerials on earlier sets of the same model (before heavyweight mass production started) often differ in length by a couple of inches, presumably as they were sourced from different suppliers.

I understand that the final set-up for such a set would include tuning the aerial output coil to produce peak radiation for the precise length of the provided aerial, and so sourcing a similar length aerial (within a few inches) should not present a problem. But I do not have any equipment which would enable me to do this. Instead, i had been machining lengths of brass rod to add to new aerials to match the original length to within a 1/6" or so. You comment that a small variation in length is not really an issue, but I am not sure what this means in practice, and sometimes my sourced aerials differ to the original length by as much as 6".

It struck me that, if the length INSIDE the Tx box were considered as part of the aerial - the part enclosed in the mount, and the mount itself, then it would be easier to simply add a few inches onto the mount to produce a precise resonant length. This would be a much easier exercise than trying to add a seamless attachment to a commercial aerial. One could simply attach a short piece of wire to the internal mount. But, of course, this would not work if it was important for the aerial total length to be outside the metal shielding box....
Pchristy
Posts: 413
Joined: 16 Feb 2018, 13:57
Location: South Devon, UK

Re: A question on aerial length

Post by Pchristy »

If you are looking at refurbishing single-channel radios, you are almost certainly talking 27 MHz. Six inches either way isn't going to make a great deal of difference for a base loaded aerial. However, as I said earlier, a centre-loaded aerial is much more critical - especially the length above the loading coil.

As for tuning, should you wish to do this, you will need a Field Strength Meter.

Field Strength Meters are quite simple hings to knock up from bits, but if you don't feel like making your own, there are quite a few available on Amazon for £25-30. https://www.amazon.co.uk/100K-1000MHz-S ... 25&sr=8-16

Don't worry about what it says about the antenna tuning it to 450MHz. Just substitute a length of copper wire! ;)

If you are going to Ponty, I've got a couple of spare FSMs made by Peter Valentine, of Lark Helicopter fame. I could fetch one along if you want one.

As long as the transmitter is crystal controlled, you don't need to worry about the frequency, just tune the output for maximum reading on the FSM, using a non-metallic tool. Most old transmitters used a ferrite slug with a simple slot in it. A piece of modelling dowel sharpened to a chisel end will do nicely. MacGregors used a beehive capacitor, but again, a suitably shaped piece of wood will do the job!

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Pete
Dodgy Geezer
Posts: 62
Joined: 17 Jun 2018, 09:54

Re: A question on aerial length

Post by Dodgy Geezer »

Thanks for the generous offer - but I'm afraid that i don't travel as much as I used to. And boats are much more my line! But knocking something up from bits sounds quite possible - do you have any recommended circuits?
Pchristy
Posts: 413
Joined: 16 Feb 2018, 13:57
Location: South Devon, UK

Re: A question on aerial length

Post by Pchristy »

Are you OK with soldering? If so, I'll have a look in my "box of bits" and see what I have. Circuit is dead simple! Are you working on 27 MHz? Some chokes have self resonant frequency of 27 or 35 MHz. If I know which you are working on, I might have one, which makes the circuit even simpler.

Won't be today, and probably not tomorrow as we're currently suffering a kitchen appliance catastrophe, which She Who Must Be Obeyed is insisting is a priority....! :shock:

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Pete
Dodgy Geezer
Posts: 62
Joined: 17 Jun 2018, 09:54

Re: A question on aerial length

Post by Dodgy Geezer »

Very much 27 MHz - sometimes without even a tone... I have a fair number of components in bit boxes over here - just need a few pointers on the best way to set up old valve sets when you're not sure about the aerial. I have a scope somewhere, but it only goes up to 10Mhz. One of these days I must learn how to use it properly... :?
Pchristy
Posts: 413
Joined: 16 Feb 2018, 13:57
Location: South Devon, UK

Re: A question on aerial length

Post by Pchristy »

Are the "old valve "sets crystal" controlled? If not, you really need a frequency counter, too! Many of those old sets that weren't crystal controlled were prone to drift quite a bit, and could be anywhere between 26 & 28 MHz! :shock:

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Pete
Dodgy Geezer
Posts: 62
Joined: 17 Jun 2018, 09:54

Re: A question on aerial length

Post by Dodgy Geezer »

Just a valve , I'm afraid. But as far as I can see there is just one adjustment available - the aerial coil slug. I'm not sure how I would go about adjusting the output frequency.
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