Multiple transmitters and close coupled antennas is a sure formula for
trouble. Some type of isolation, either physical by distancing
antennas, or electrical by an isolator will most likely be required.
Isolation of antennas by distance (in terms of several hundred feet or
more) is much less expensive than electrical isolators which can cost a
couple of grand per frequency.
73
Bob K4TAX
Bill Miner wrote:
>
> Jim,
>
> I have been watching your posts regarding this project and I do have one
> caution for you! If you do not have enough isolation between
> transmitting antennas it is possible to get external RF back into the PA
> of any of your transmitters and that PA will act as a high level mixer.
> One symptom of this could be an increase in SWR. At VHF/UHF and
> microwave frequencies the fix for this is the use a circulator or
> directional coupler on the output of the transmitter. In your case I
> believe the easiest method would be to make sure you have plenty of
> distance between transmit antennas. This would mean
> no antennas on the SAME boom! The most common of the mixing products is
> 2A+B or 2A-B. As I am sure you will see.... some products will be
> outside of the ham bands and may get you into possible trouble with
> some government agency!
>
> I believe a good source for additional information on this subject would
> be someone at W1AW since they have lots of experience with multi
> transmitters. Talking to W1RFI might be a good place to start.
>
> I hope this is helpful and I hope you are successful with your project.
>
> 73,
> Bill - K6WLM
>
> Jim Reid wrote:
> >
> > Hi,
> >
> > I am now keying three TT rigs simultaneously; an Omni VI+,
> > on either 20 or 15 meters feeding a C3; a Triton IV (544)
> > on 40 feeding a F12 rotating dipole mounted on the SAME
> > boom with the C3, and a Corsair II operating on either 30
> > or 17 meters feeding a doublet wire inverted V, the apex of
> > which is about 10 or so feet below the boom holding the C3
> > tribander and 40 dipole (F12 C4 set up).
> >
> > Purpose is putting out CW practice on three bands at the
> > same time (similar to W1AW). Now what I am seeing and
> > possible problems. I presume each rig is transmitting within
> > very few milliseconds of one another as the same keying
> > signal goes to each. All seems to be operating as intended,
> > I have gotten good reports from across most of the US
> > mainland.
> >
> > What I see: the S/vswr meter on the Triton moves significantly
> > on transmit. It should not, the vswr on it's transmission line is
> > 1.1:1; and it if is keyed independently, the meter needle
> > moves not a whit. But if the Omni is keyed on 15 meters at
> > the same time (remember both radiators are on the same boom)
> > the Triton S meter jumps to a couple points, so the Triton rcvr is
> > "hearing" the Omni signal, I presume. If only the Triton and
> > Corsair are keyed (Corsair on 17 meters, say), the Triton
> > S-meter does NOT move a bit.
> >
> > Second, I notice after the rigs have been keying away for
> > an hour or so, the digital readout on the Corsair begins to
> > behave erratically -- indicated frequency jumps up a kHz or
> > so occasionally, again when the Omni is keyed along with
> > it. This does not occur on initial turn on for awhile. Is
> > the Corsair actually changing the transmitted frequency,
> > or is this indicator instability?
> >
> > I will be doing this again today at 2200 and 0030 UTC in
> > about 3 hours and 5 1/2 hours from now. Would appreciate
> > any signal reports on what you might hear at 18097.5 kHz
> > and 21067.5 kHz. Doubt if the 7047.5 signal will be
> > heard on the mainland at either time; it is for KH6 land
> > practice.
> >
> > Thanks in advance for reports or thoughts on this situation.
> >
> > 73, Jim KH7M
> >
> > _______________________________________________
> > 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
|