Coax length affecting reception

[veedub565]

Hi,
I built myself one of those chinese airband kits you see on ebay. It's all assembled into a metal project box and working well.

The problem I have is that it works well with a 5m length of coax, but when using a 1m length then the reception is quite poor. Which seems a bit odd, I'm guessing maybe something to do with swr and matching?

The antenna itself is a homebrew dipole at the appropriate length for civil airband.

Really strange I seem to get more gain if you like with a longer feed coax

[Paul Rose]

I had a peek at e-bay.  This look like a fun kit with a well designed circuit.  I might have to get one

Incidentally, the circuit looks like it is from Tony van Roon's web site: http://www.sentex.ca/~mec1995/circ/aviarx/aviarx.html

It could be a impedance issue.  A length of coax will transform the impedance of the load as seen by the source if the impedance of the source, load, and characteristic impedance of the coax don't all match.

This can actually be put to use for matching: 
  Zo = sqrt( Zs * Zl )
Where Zo is the characteristic impedance of a 1/4 wave long piece of matching coax, and Zs and Zl are the source and load impedances.  This is awkward to use, however, because usually Zs and Zl are set in stone, and you can't just order up an arbitrary impedance Zo coax to fit your need.


Is your dipole acting like a dipole?  Can you get a peak/null on a distant source by rotating the antenna?  If not, then it may be that the whole feed line is acting as part of your antenna.  It could be a balance issue.  A balun at the feed point of the dipole might help here.  A receive only balun at this frequency should be easy to achieve, even an old VHF TV balun might be OK.   

[veedub565]

Thanks Paul,

Been looking into it this morning and I think it may be a dodgy BNC connector on the kit. When I fitted the kit into a project box I used a bulkhead BNC salvaged from an old piece of equipment.

I inserted the 1m length in between the receiver and the 5m length and the signal dropped right off. I tried 3 different 1m BNC leads and result was the same. I reattached the 5m length directly to receiver and jiggling the connector I was able to make the signal strength come and go. The connector on the 5m length must be making better contact than the others.

So new BNC connector on order.

I did try rotating the antenna but I was getting some strange effects due to the proximity of my hand, probably due to the dodgy BNC. I'll have to read up on Baluns.


The kit has been fun to build, and a learning experience, which was partly the point. I could have bought a cheap readymade airband receiver for £50 or whatever, but this has been much more fun. I made some mods to improve it from standard too