TRX2A Power out falls off, Sub Harmonic.

General discussion

TRX2A Power out falls off, Sub Harmonic.

Postby KH2BR » 06 Jan 2012 03:01

After my first cw contact, I noticed that the power output was falling down. The back of the radio was very hot and I think
that is because of the back plate its self. It is not like the one I see in the pictures. My back plate is made of
double side PC board that has holes in it. I think the heat sink properties of that plate are insufficient. I was able to fix
it by attaching a small fan to the back plate. Now, the power output is stabilized.

I was listening to 160 meters and tuned 3 kHz above the band and heard a shortwave broadcast talking about amateur radio.
I thought, How nice. I checked with another radio and that signal was not there. I read somewhere about Tayloe muxers and sub harmonics and checked 6.009 MHz. There was the station. Is there a way to prevent Sub harmonic reception?
How about reducing LO input drive to the muxer? Will that help or will it cause other problems?

I am very pleased with the radio and enjoyed building it very much. I have made many contacts with it already.
I have a Icom IC-7600 but enjoy the TRX2A better because I built it.
The layout and design are first class. Thanks for such a nice experience.

Robert KH2BR
KH2BR
 
Posts: 32
Joined: 13 Nov 2011 07:01
Location: USA

Re: TRX2A Power out falls off, Sub Harmonic.

Postby OH7SV » 06 Jan 2012 19:25

Robert,

The TRX2 rear panel has been changed to double sided PCB with thick copper layers abt one year ago. You can feel heat but that is ok because the final MOSFETs can easily handle the power dissipation although they are warm. 2 ea IRFI510 mosfets can dissipate more 40W at case temperature of 60ºC. The main reason for power fall off is the intentional bias over compensation which will lower the PA gain. In desibels it is not much, if you loose power from 10W to 8W it is only 1dB = 1/6 of S-unit.

I think you were listening at frequency 2.003 MHz. Thus your 6.009 MHz spurious finding is not a sub harmonic. It is the third harmonic of the fundamental. Mixer 3rd harmonic principle response is 9.5 dB below the fundamental response witout additional filtering. When passimg 2 Mhz the TRX2 RF filters will change to the 3.5Mhz band (Filter BPF 2MHz...4MHz). The upper corner of this filter is quite near the 6.009 MHz signal you were hearing. TRX2 RF filters are opmized for amateur use and you found the worst combination of the filters on 2.003 MHz and you had a strong BC station at 6.009 MHz. If you stay below 2 MHz the situation is much better because the selected BPF is on range 1MHz...2MHz. And when you tune to 3.5 MHz the 3rd harmonic response is at 10.5 MHz and the situation is also good because the 2MHz...4MHz filter will give good attenuatuon on 10.5 MHz.

Nice to hear that you like to work with JUMA TRX2. Enjoy QSOs :D

73 Matti OH7SV
User avatar
OH7SV
Moderator
 
Posts: 107
Joined: 11 Nov 2011 07:00
Location: Finland


Return to Juma Discussion

Who is online

Users browsing this forum: No registered users and 11 guests

cron