Interesting Quick Thought

Tony Langdon tlangdon at
Mon Jan 21 12:44:13 EST 2002

> seen people use a shielded relay (about $55) to switch so 
> that when they
> transmit on one radio it switches the aerial to the transmit 
> section, and

You would have to use a diplexer to route the 2.4 GHz to the WLAN card and
the lower frequencies to the TV.  This would probably involve a couple of dB
loss at 2.4 GHz (unless you get some _really_ good filters and connectors).

> Also, recieving and transmitting are a different kettle of fish. For
> transmitting you need a multiple of the wavelength. 
> Preferable is exactly
> one wave length, but if size is/isnt a contraint you can half it, and

This is an old CB wives tale.  Coax length is generally unimportant (notable
exceptions are HF tuned feeder antenna systems and transmission line
matching sections (the latter use precisely cut lengths of feedline to
achieve impedance transformations).

2 points on this matter that apply to most radio systems (except those
mentioned above).  Firstly, a 1/2 wavelength of coax is an "impedance
repeater" meaning the impedance at the load appears also at every 1/2
wavelength back along the coax.  This doesn't generally aid matching in most
normal circumstances.

If your VSWR (which is an indicator of how well the system is matched)
changes with length of coax, then you have a problem with either your
antenna system (causing stray currents on the outside of the coax) or your
SWR meter.

Personally, with all the mismatches in the system, I think one would be
better off putting a modified Galaxy antenna or similar up the mast.  Better
performance and lots less hassle.  As for cost, the really expensive but is
the network card itself in any case.

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