[hpsdr] Using Mercury to monitor final amplifier transmit signal

Sun Feb 10 15:33:08 PST 2013

Also check http://www.cleanrf.com/products.html#RF-SM
73 ... Sid.

On 10/02/13 22:55, Dick Faust wrote:
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>
> 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
>> ***** High Performance Software Defined Radio Discussion List *****
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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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