Hi Curt:
Yes, you recollect correctly. 17m is not it's strong point. If you look in the
archives there's an extensive discussion, the best post from which I've copied
below. Mine has all the WARC crystals so I've used it. Worse than birdies in
the receiver it transmits spurs, perhaps 10 or 20 db down. Far as I know, no
solution has been found except to use an auxiliary transverter. Too bad because
17m is one of my favorite bands.
In addition, mine has all sorts of noise from the frequency counter on 17m
(only) and I haven't found a solution to that.
The description below is correct except that he says the output of the VFO is 9
to 9.5 mhz...the VFO actually runs at 5 MHz but is mixed to the 9 MHz range so
the effect is the same. The mixing then proceeds as Steve describes. He's
written extensively on the unit, you can find the mods at the Ten-Tec Wiki site.
tnx es GL,
-Pete
WB2QLL
Somers, WI
To: <tentec@contesting.com>
Subject: [TenTec] How the Omni C produces spurs on 17meters
From: N4LQ@iglou.com (Steve Ellington)
Date: Thu, 23 Mar 2000 21:35:08 -0500
I'm replying to everyone because there is some question at to how the Omni C
produces the nasty spur on 17 meters. Here's how:
When operating on 17m, the output of the VFO runs between 9 and 9.5mhz.
This signal is then fed to the TX mixer where it is mixed with the 9mhz,
carrier from the BFO. Mathematically, we have the sum of the two
frequencies, 18mhz to 18.5mhz and the difference, zero to .5mhz.
This zero to .5mhz carrier mixes with the desired signal and as you can see,
the difference in the two frequencies is small. For example, when we operate
on 18.070mhz, the VFO supplies 9.035mhz which is mixed with the BFO to
produce both the desired frequency AND .035MHZ.
Now the trouble starts. Here we have a healthy .035MHZ carrier which mixes
with the 18.070mhz desired signal to produce 18.105mhz and 18.035mhz. So we
now have two carriers within the 17m band and one that isn't. Lets move the
dial on up to 18.150mhz and now our spurs are at 18.225mhz and 18.025MHZ.
So the whole problem here is due to the VFO and BFO being much too close in
frequency thus producing products too close for the transmitter filter to
remove when operating on 17 meters. It's a bad design to say the least. On
other bands, the undesired products are several Mhz away from the desired
frequency and easily filtered by the TX. Just think; If we could operate on
18.001mhz, we would QRM ourselves!
Steve
N4LQ
-----Original Message-----
>From: Curt <rhulett1@consolidated.net>
>Sent: Nov 22, 2011 1:35 PM
>To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
>Subject: [TenTec] Omni C not for 17M?
>
>Recently acquired fairly nice Omni C, a model which I have never owned. It
>lacks crystals for 12 and 17M, recall reading posts stating that 17M
>wasn't useable on this rig because the mixing scheme allowed it to put out
>birdies violating spectral purity regs. I'm recalling mention of birdies
>1-2W on 17M (?!)
>
>Just wanting to confirm that this issue was never corrected, and that
>operation on 17M must be avoided.
>
>73, Curt KB5JO
>
>_______________________________________________
>TenTec mailing list
>TenTec@contesting.com
>http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|