On Thu, 2006-06-01 at 21:32 -0700, Jerry Volpe wrote:
> I am selecting which of my transceivers to take to Field Day with the
> choices either my Corsair II or my Omni-VI+. You would think this to be
> a slam dunk decision in favor of the newer radio. However, I still have
> a very bad memory from last Field Day when I took a Kenwood TS-940SAT
> which had so much Phase Noise that when transmitting I wiped out the
> other two club stations. I have subsequently deep six'd that
> transceiver. Now not wanting a repeat of that experience I decided to
> make a very simple test.... and got rather disturbing results.
>
> Test One: Omni-VI+ to a dummy load. 14.005 MHz. Power output varying
> between low and hight power. Keyed with electronic keyer. Corsair II
> with unterminated receiver (basically on a shorted connector of the same
> switch connecting the Omni-VI+ to the dummy load). As I tuned across
> 20-meters I could not hear the Omni-VI+ transmitting until I was pretty
> much on the transmit frequency +- say 3 kHz. This was satisfactory.
> Changing the Corsair II's XTAL filter to 1.8, 500, or 250 Hz made the
> reception tighter as expected.
>
> Test Two: Reverse the roles. With the Corsair II transmitting at even
> the least output possible the Omni-VI+ presented 'keying' noise across
> the entire 20-meter band with a few false signals to boot! Inserting
> different first or second IF filters did little to reduce this.
> Eeeaaoooouuucchhhhh! Granted the noise floor alternately raising and
> lowering with the keyed Corsair was maybe an S-3 to S-4 level but having
> that noise constantly present for 24 plus hours..... Hmmmmm.
>
> To be sure that this was NOT the Corsair II's problem I turned on
> another transceiver..... an Argosy with an 8-pole first IF filter. The
> Argosy's response to the keyed nearby transmitter was even better than
> that of the Corsair II's!! Hmmmmmm, something to be said for
> single-conversion designs.
>
> I got on the phone and talked to a tech at Ten-Tec about this. He did
> NOT indicate that this was abnormal for the Omni-VI+. He did state that
> the Corsair II was arguably the finest 'analog' transceiver ever
> made..... still he recommended my using the Omni-VI+ as he believes it
> to have a better receiver. Now this is something I have never been able
> to confirm. I said what about the Omni-VI+'s susceptibility to the
> nearby transmitter? Long pause..... No comment.
>
> Next I asked him if he thought adding the INRAD roofing filter might
> help. I had my doubts as the interference was felt across the entire
> 20-meter band! Knowing that the Omni-VI+ has a 16 KHz monolithic crystal
> roofing filter already I wasn't sure how putting a tighter filter would
> help. Still I am grasping at straws here.
>
> He did say that a number of INRAD roofing filter equipped Omni-VI+
> transceivers have found their way to Ten-Tec and that there was a
> 'noticeable improvement'..... and that INRAD filters have been found to
> be of very high quality.... Yada yada. Would it help with what I am
> experiencing? No way to know.
>
> So what do you think? Have you installed an INRAD roofing filter in your
> Omni-VI+? Which... 2.4 or 500 Hz? And did you use your
> unmodified/modified transceiver in a multi-contest environment?
>
> Thanks for the input.
>
> Jerry, KG6TT
>
I'd use the Corsair II. No question, but then I don't own an Omni-VI.
And I chose the Corsair II over Omni V or VI because they run CW on LSB
and I intended to use the Tentec as an IF radio with transverters for
VHF where contacts are made cross mode and having the radio shift to LSB
for CW causes too much pain and too many lost contacts. The Corsair II
uses USB for CW which suits me much better.
The RF circuits of the Omni and the Corsair are similar in concept, but
different in details. The Corsair uses a transformer feed back circuit
with a small RF power transistor (made for cable TV) for the RF stage
and the first stage after the first mixer (to provide a broadband mixer
termination to make the mixer work better). The Omnis use 4 JFETS in
parallel grounded gate. I've not seen a direct comparison of how they
handle strong signals. The Corsair's circuits are the favorite ones of
Ulrich Rhode, the receiver guru. He never has mentioned 4 JFETS in
parallel in his books that I've found. There once was a large JFET that
showed promise but its not been available for decades so its hard to
design into a product. I expect the four FETS in parallel are an
approximation of that.
You might also compare the effects of the RF "attenuator." In the
Corsair it actually takes thee RF stage out of the circuit. I don't
remember what that switch does in the Omni.
The tunable oscillator in the Omni is synthesized, but in a manner that
keeps the phase noise reduced, while the analogue PTO of the Corsair
could still have much lower phase noise.
For our club FD last year I separated the stations by a bit over 500
feet and added in low and high pass filters to keep the 80/40 station
from bothering the 20/15 station but allowed only one radio at each
station so there were never two stations on the same band and location
and so the ICOM 746 on the low bands and the FT-857 on the high bands
were never bothered by the other station.
I don't think the narrow roofing filter for the Omni VI will solve your
problem. I think using the Corsair II will be more fun and more
productive though it won't think for itself, nor log automatically.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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