Antenna Feed Line

[richard]

I use a simple untuned wire loop connected to an FET amplifier. I am currently using 1m of audio cable to connect the amplifier to the receiver. The receiver is a Degen 1102, which uses a 3.5mm audio socket. I am guessing that proper RF cable would make a better feed line but does anyone else use audio cable, is there significant loss?

Cheers, Richard

[dk8cb]

In general, attenuation in a connection (coaxial cable) between source (amplifier output) and load (receiver input) depends on a number of facts:

the length of the cable
the degree of mismatch at the source and the load
the properties of the cable.

With the receiver input of portable receivers usually around 500 ohms, you are not using a matched system, ie your amplifier, cable and receiver input certainly won't all have the same impedance and it is the imedance mismatch which will cause the largest part of the loss. Since the cable is very short in wavelengths, it's influence will not be significant, except if your amplifier has got a fairly high output impedance. In such a case, the cable will just act more or less as a capacitive load. But using a piece of RF coaxial cable will not make a noticeable difference in such a case.
My advice: Leave it as it is, there won't be a difference.

One might argue that a 50 ohm amplifier output could be transformed to the receiver input impedance by an RF transformer to increase level, however in the real world, it is much more important to maximise the ratio of wanted signal to unwanted signal (noise) than to increase the level without changing the ratio which - in case of broadcast reception - is only rarely limited by receiver noise but by other factors. But replacing the audio cable by RF cable will not increase your level at the receiver input anyway.

Roland

[richard]

Roland, thanks for this advice. Think I will leave it as it is then.

I noticed that when connecting the loop through the cable to the receiver the loop appeared to be tuned, so the cable must have been acting as a capacitive load, as you described. I built the amplifier to try and overcome this problem.

With modification I think I could probably get the small loop I use to work better untuned, maybe using multicore cable to add more turns and hopefully increase signal strength. This is partly my intention because I find it difficult to get the capacitance/inductance right to cover the whole HF range. Also less fiddly when changing channels. At least I know now that the cable is not the severely limiting factor I thought it might be.

Richard

[dk8cb]

With modification I think I could probably get the small loop I use to work better untuned, maybe using multicore cable to add more turns and hopefully increase signal strength.

If you want an optimum result, then you shouldn't use multicore cable because the close proximity of the turns will increase the wire resistance and reduce efficiency. If you place the turns at a distance of a centimetre or so, the wire resistance (mainly caused by the skin effect) will be smaller.

With normal portable receivers which usually cannot handle strong signals well, it is advisable to prefer a tuned antenna over of a broadband type because performance with large signals from unwanted frequencies which cause undesired products in the receiver, can be improved a lot and signal to noise ratio can usually be improved quite a bit.

Roland

[richard]

Have seen an equation for skin effect but this does not take into account the spacing between wires- Maybe this assumes an ideal case were there is no interaction. In an untuned loop I assuming that that a small loss resistance is not going to be a much of a problem, as you are not too bothered about achieving maximum Q. I agree that a tuned loop would be beneficial for better front end selectivity though. Think I am lucky in that my receiver does not appear to suffer much from overloading.

I have also been experimenting with winding an antenna on a type 61 ferrite rod from CWS ByteMark (http://www.cwsbytemark.com/prices/rods.php) in the US. This has a lower permeability than normal MW/LW rods. Appears to be able possible to tune this arrangement over the HF range with a few turns of wire and a variable capacitor. So may decide to ditch the air core loop idea and use the rod instead!

Richard