EMRFD Message Archive 6838

Message Date From Subject
6838 2011-11-17 14:52:10 Bill Meara My JBOT hates antennas
I built a JBOT amp.   It works fine into a dummy load, but when I connect it to an antenna, it gets unstable.  Here are some more details of the symptoms:

I am running the JBOT with a 5 element (two toroids, 3 caps) low pass filter (designed by Doug DeMaw)

With the antenna connected, all is well UNTIL I raise the power out (by varying the input) beyond about 1 watt.  Below one watt, the amp is working fine, and it stable.  As soon as I hit the 1 watt point, the amplifier seems to break into oscillation.   This does not happen into the dummy load.

The antenna is a simple dipole fed by coax.  It shows a low SWR.  Even when I put an antenna tuner between the amp and the antenna and bring the SWR down to negligible levels, the instability problem persists. 

With the amp disconnected from all other circuitry other than the antenna and the power supply, if I just touch the input capacitor, it breaks into oscillation. This does not happen when the amp is working into the dummy load.

I've bolstered the power supply filtering and decoupling.  No luck.  I tried some de-Qing of the transformers. No luck.

Any suggestions?

Bill N2CQR

[Non-text portions of this message have been removed]
6839 2011-11-17 14:56:38 dan edwards Re: My JBOT hates antennas
are you using a balun at the feedpoint?   73, w5xz, dan

6840 2011-11-17 16:07:39 Jim Miller My JBOT hates antennas
Inspite of SWR you could have common mode current. Likely if a dipole fed
by coax. Goog K9yc for explanation and cookbook choke solns. Goog w9cf for
math details.

Jim ab3cv

[Non-text portions of this message have been removed]
6841 2011-11-17 16:17:51 Bill Meara Re: My JBOT hates antennas
No, from the secondary on the output transformer I go to the LP filter and then to the coax.

6842 2011-11-17 17:20:06 dan edwards Re: My JBOT hates antennas
i'm thinking about a balun at  the far end of the feedline, at the antenna...sometimes the shield of coax can be 'hot' with r.f....sometimes, odd feeline lengths can do strange things too...just a thought...best of luck....73, w5xz, dan

6843 2011-11-17 17:56:54 Ashhar Farhan Re: My JBOT hates antennas
the swr could be low and still reactive. You could try some more variations -
1. Do the oscillations happen even without the low pass filters?
2. What is the frequency of self-oscillations?
3. Have you tried an attenuator at the input of the amp?
4. Is the entire chain oscillating? You could add a 6db attenuator at
the output of the predriver.

Oscillation will happen when there is a 360 degrees phase-shift with
gain. It means that there are some reactive elements coming into the
loop when you connect the antenna.

One temporary fix is to use an atu. But we need the amp to be less
sensitive than that. this needs a bit of detective work.

There are two texts that deal with this - one is by rick campbell (I
forget the title, though it is available as a pdf on the net). The
other is demaw's 'how to tame a transmitter'.

Best of luck with the efforts. This week has been a frustrating one
for some of us, radio wise. Our friend hans nearly wrecked his
glow-bug vfo trying make it oscillate. I lost a whole lot si570 code
in a disk crash.

- farhan VU2ESE

On 11/18/11, Bill Meara <n2cqr@yahoo.com> wrote:
> No, from the secondary on the output transformer I go to the LP filter and
> then to the coax.
> ---
6844 2011-11-17 18:18:21 Alex P Re: My JBOT hates antennas

Have you looked at including some neutralization?


6845 2011-11-17 18:38:35 w1kilofoxtrot Re: My JBOT hates antennas
Just for testing, does the amp oscillate if you leave out the LPF and connect directly to the antenna?

It almost sounds like there is a phasing error in the transformers you've wound, which is exaggerated by whatever reactive load the antenna presents. You might wish to double check those connections, to make sure you haven't swapped one...


6846 2011-11-17 19:21:33 kb1gmx Re: My JBOT hates antennas
6847 2011-11-17 21:57:30 Hector Pascal Re: My JBOT hates antennas
If all else fails, I'd be looking at improving the input match with a
small resistive attenuator (as suggested by Farhan, note #3).
I've known amp designs that were very tricky about their input match
when the layout was less than perfect.

David, VK6JT

6848 2011-11-18 03:44:07 Bill Meara Re: My JBOT hates antennas
Thanks for all the good advice and encouragement on my JBOT problems. 

I tried the choke/balun on the coax.  No improvement.   But I think Allison and others are right in saying that this is a grounding/shielding kind of problem.  My next move is to tighten things up in this area.   

BTW this amp is going into a 17 meter DSB transceiver that I first built at about this point in the previous solar cycle.  See here for details:


73  Bill N2CQR