I want to add preamps to my VHF/UHF setup for weak signal work. During some
reading, I found out it is not recommended to place an external wattmeter
between the transceiver and the preamp (mast mounted). They said that it
could interfere with the operation of the preamp

Can someone explain this to me?

Some preamp get there power from the coax and the watt meter would dc
ground the coax, therefor no voltage to the preamp.




On Tue, 12 Sep 2006 17:50:11 -0700, "Rod Maupin"
wrote:

I want to add preamps to my VHF/UHF setup for weak signal work. During some
reading, I found out it is not recommended to place an external wattmeter
between the transceiver and the preamp (mast mounted). They said that it
could interfere with the operation of the preamp

Can someone explain this to me?

"W2RAC" wrote in message
...
Some preamp get there power from the coax and the watt meter would dc
ground the coax, therefor no voltage to the preamp.

Also be aware that unless you place the preamp at the antenna you are
unlikely to obtain any system improvement unless the NF of the preamp is
better than the rig's front end NF.
By placing the preamp at the antenna it will overcome/mask the loss in the
coaxial cable and system NF will be determined by the preamp.
Dale W4OP

On Wed, 13 Sep 2006 12:37:12 GMT, "Dale Parfitt"
wrote:


"W2RAC" wrote in message
.. .
Some preamp get there power from the coax and the watt meter would dc
ground the coax, therefor no voltage to the preamp.

Also be aware that unless you place the preamp at the antenna you are
unlikely to obtain any system improvement unless the NF of the preamp is
better than the rig's front end NF.

"Unlikely to obtain any system improvement" is a gross simplification,
and not true in most cases.

By placing the preamp at the antenna it will overcome/mask the loss in the
coaxial cable and system NF will be determined by the preamp.

Another oversimplification.

Back to the original question, if inserting a wattmeter added 0.1dB
loss for instance, the effect on system Noise Figure of insertion
prior to the preamp will be quite small, even for the hottest preamps.

The argument by another person that wattmeters shunt the DC feed to
the preamp is also a gross generalisation, most VHF/UHF wattmeters do
not interrupt the DC continuity, and do not shunt the line with a DC
path.

Elsewhere, I have seen an argument that the diodes in the direction
wattmeter sampler create substantial intermod products that are
re-reradiated and detectable nearby. I have never seen a workup of
numbers that supports this, and my own estimates are that it is
probably insignificant.

Owen
--

W2rac wrote;
'---the watt meter would dc ground the coax---."

Not the Bird wattmeter and its clones.

Best regards, Richard Harrison, KB5WZI

Thanks for all the replies. I will be mounting the preamps on the mast,
close to the antenna.