[NLRS] Downconverters

[Dr. Gerald N. Johnson, electrical engineer]

john scherer wrote:
> 
> Hello all,
> <SNIP>
> 
> I could do either.  Intuition tells me that if I can
> receive at 23cm, and the run to the antennas is
> reasonable, not to use the downconverter.  Anytime you
> insert something into the signal path you incur
> losses, the case for the downconverter seems to be:
> which loss is greater, the loss signal at 23cm due to
> cable length or the loss of signal due to addition of
> downconverter.
> 
> Is this sound thinking?
>
No. The MDS and phase noise of the best rice box is very poor compared
to a decent down converter whether located in the shack or at the
antenna.

Many using satellites keep their antennas close to the shack so that
feed line loss isn't so much of a problem, but universally they use a
low noise preamp AT the antenna because the signals from the satellites
are on the weak side and once attenuated into the thermal noise of the
antenna and receiver inputs, there's no amount of signal processing that
can improve the S/N. S/N only gets worse in the typical receiver from
excess mixer noise and reciprocal mixing of phase noise.

Using less expensive coax from downconverter to receiver in the shack,
helps to compensate for the bother of installing and maintaining the
converter at the antenna (especially when its on top of the tower).

Look at 3/4" CATV hardline. Connectors are reasonably priced, and the
cable is often free because the cable company won't splice it between
poles, so they tend to have roll ends over 100' long. The modern cable
has less loss at UHF than the older style and I found the older style
has less loss at 1296 (by actual measurement being careful to not let
the impedance mismatch confuse the measurement) than 1/2" foam heliax.

As W0ZQ says, if you don't have G3SEK's book, the VHF/UHF Dx Book, you
need it. ARRL stocks it or you can look it up on Ian's web page.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.