Desensing issue


g0ifl
 

Hi,
If I run a little extra power (ie 200W+) my receive signal drops.
I see this on the P3 as the noise floor drops and also in audio with AF drop.
Once the tx power is removed, a slow (meaning approx 2 sec) restoration of noise floor/audio is achieved.

Not having much space to play with (living in an apartment) my radio/amp are closely spaced and the antenna is approx 5m from me.
Antenna is new, coax is new and not considered as at fault as, with the previously installed antenna, I had the same issue.

I never run more than 500W.

I assume the closeness of the the antenna might be the issue but wonder whether there maybe something else I should be considering.


Using a K3 (8415) which is pretty much fully loaded (excl 2m module) and with no K3S mods. SPE-1K Amp and a Cobwebb antenna (3KW version)

Tnx in advance


Rob
HB9ETR/G0IFL


Ray Maxfield
 

Rob.

Try Transmitting into a Good Quality Dummy Load for a Test.

 

Ray WA6VAB K3

 

 

From: g0ifl via groups.io
Sent: Saturday, July 10, 2021 8:26 AM
To: Elecraft-K3@groups.io
Subject: [Elecraft-K3] Desensing issue

 

Hi,
If I run a little extra power (ie 200W+) my receive signal drops.
I see this on the P3 as the noise floor drops and also in audio with AF drop.
Once the tx power is removed, a slow (meaning approx 2 sec) restoration of noise floor/audio is achieved.

Not having much space to play with (living in an apartment) my radio/amp are closely spaced and the antenna is approx 5m from me.
Antenna is new, coax is new and not considered as at fault as, with the previously installed antenna, I had the same issue.

I never run more than 500W.

I assume the closeness of the the antenna might be the issue but wonder whether there maybe something else I should be considering.


Using a K3 (8415) which is pretty much fully loaded (excl 2m module) and with no K3S mods. SPE-1K Amp and a Cobwebb antenna (3KW version)

Tnx in advance


Rob
HB9ETR/G0IFL

 


Stewart G3RXQ
 

Rob,
It might be a problem with the relay contacts in the SPE-1K amp. The original relays were not up to the job.
The symptoms describe the issue where the receive path is affected by poor relay contacts.
The original relays are ones manufactured in Italy. Changing them for Finder relays cures the problem .

I suggest that you join the groups.io SPE group and have a look at some the posts on the subject.

73 Stewart G3RXQ / G3Q


Bob McGraw - K4TAX
 

As suggested, test with a known good quality 50 dummy load.  If the issue does not appear, then I would consider adding some ferrite chokes on the coax line at the K3 output.   If the issue appears with using the dummy load then as suggested, the relays in the amp may be at fault. 
 Also wouldn't hurt to add some on the DC power lead as well.   With an antenna that close there is a high likelihood of high RF levels at the operating position. 

73
Bob, K4TAX


g0ifl
 

So, after taking the advice from group members...

I find that transmitting 400+ Watts into a dummy load does not reproduce the symptoms.
The noise floor is constant and the AF doesn't drop at all.
I presume that removes the radio, the amp and everything connected between out of the equation.

Next test will involve earthing before I am left with the assumption that the relatively closeness of the antenna causes my issue.


Jim Brown
 

On 7/17/2021 1:45 AM, g0ifl via groups.io wrote:
Next test will involve earthing before I am left with the assumption that the relatively closeness of the antenna causes my issue.
Here's a serious take on that. Note that a few details of the bonding of the protective earth to neutral is different in the UK. EVERYTHING else applies.

http://k9yc.com/GroundingAndAudio.pdf

Also, study this tutorial on RFI and ferrite common mode chokes. k9yc.com/RFI-Ham.pdf

73, Jim K9YC