[hpsdr] Using Mercury to monitor final amplifier transmit signal

[Dick Faust]

There use to be a construction article in the older ARRL Handbooks for a 
non contact sampler made from a copper pipe T fitting. This was based 
upon a Bird 43 through line Line Section Model 4230-53 and the Model 
4274 Sampler elements which work very well but only offer limited range 
of frequency and attenuation. I have this setup and have seen 
[someplace] a discussion on the design of the sampler elements.

I also have a RF Tap-off from Motorola Model # SLN6321A made by Coaxial 
Dynamics Inc of Cleveland OH which is an adjustable version and much 
less expensive. Also most likely a collectors item.

Coaxial Dynamics still offers tap-offs on their web page:
http://www.coaxial.com/products/7900_series.htm

This style of tap-off offers a lot of flexibility and isolation as 
additional coaxial attenuators can be attached to the sampling port. If 
you initially use a switchable type like the AT1 Step Attenuator offered 
by Elecraft, you can find the right amount of attenuation for your setup 
and then just make a fixed in line attenuator for your shack. They also 
permit sampling directly from the antenna feedline with out upsetting 
the SWR.

I used to use these and similar products to protect the old style 
Spectrum Analysers which were most unforgiving to high power RF.

Dick Faust K9IVB

Tom Cathey wrote:
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> Jean-Claude,
> What Joe said.
> I think you need to find your own sampling level “sweet spot.” Dynamic 
> range requirements are dependent on how low into the noise you need to 
> see the crud up the band. If the signal is too strong, you may 
> generate spurious. If it is too weak, you will not see the stuff that 
> is way down near the noise floor. Experiment with pickup levels and 
> see what works best for your particular set up,
> I found that just the SO-239 jack “inner stub” mounted on the sampler 
> chassis is all that was needed to sample enough signal for the Mercury 
> DUP function. I tried a ferrite xfmr and that was way too much signal. 
> In addition, I added a 10 turn 2K pot for a variable atten at the 
> change-over relay to give a wide range of signal level coverage – when 
> using from 10w to 1500 w of power output.
> Also be careful with stray pickup in the Merc changeover relay box. I 
> needed to use beads and .001 bypass caps for the DC relay leads. They 
> were bringing in an extra 20db of stray pickup when unfiltered.. You 
> should be able to disconnect the sampler coax and see the transmit 
> signal drop way down, like at least -30 dB or more. Otherwise there is 
> stray pickup around the intended sampler box. Draw the plan out and 
> you will see what I mean.
> All in all, this new “DUP” IMD monitoring system has become my main 
> station monitor – in addition to a standard o’scope. Nothing like 
> watching a spectrum analysis of our overall FINAL transmitted signal 
> on the air. But it must be a signal that is sampled without stray 
> pickup or it will be invalid.
> When testing, you will find there are amplifier drive, loading, 
> biasing and reduced power output “sweet spots” that give better IMD 
> than others.
> Tom, K1JJ
> *From:* Jean-Claude JCA. Abauzit <mailto:jca at cssbv.com>
> *Sent:* Sunday, February 10, 2013 10:51 AM
> *To:* hpsdr at lists.openhpsdr.org <mailto:hpsdr at lists.openhpsdr.org>
> *Subject:* [hpsdr] Using Mercury to monitor final amplifier transmit 
> signal
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> What is the safe level in dBm to use when monitoring the transmit 
> signal? Here I have Hermes and I suppose the max level on RX is the 
> same as for Mercury.
>
> Thanks,
>
> Jean-Claude PJ2BVU
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