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Author Topic: Help make sense of desense  (Read 2475 times)

Posts: 4

« on: July 13, 2012, 05:03:38 PM »

Hi all,

When using a 2m repeater, I would like to monitor the output frequency on a scanner at the same time.
To be able to hear myself on the repeater would be great, but the scanner becomes desensed(?) due to the close proximity of the radios.

I can't move the scanner to another room because I wouldn't be able to hear it. Not to mention feed lines, bothering others in the house, etc.
From what I've read, it seems like I need some kind of filter for the scanner? 

Since this isn't an absolute necessity, I'm looking for a cheap and simple solution.

Any suggestions o' wise Elmers?




Posts: 1044

« Reply #1 on: July 13, 2012, 06:07:28 PM »

A simple experiment is to remove the scanner's antenna and/or replace it with a dummy load.  It's just a shot, but it might work.  Another idea, make an extra long headphone cable.... GL

Posts: 13017

« Reply #2 on: July 13, 2012, 06:13:11 PM »

Think about it - that is similar to what a repeater has to do:  listen to a signal on one
frequency while a transmitter is putting out full power a few hundred kHz away
without desense.  (On 2m, anyway - the spacing is wider on 440.)

To do that, a repeater typically requires an expensive set of cavity filters, one set
tuned to the receive frequency with maximum rejection on the transmit frequency,
and a second set for the transmitter with maximum rejection on the receive channel.
Both transmitter and receiver need to be well shielded, because any stray RF pickup
through DC wiring, speaker or mic cables, etc. will couple unwanted signal from one
to the other.  It is also important to use a transmitter that has minimum spurious
signals on the receive frequency, and a receiver that can handle strong signals
without overloading.

Now, things are a bit easier in your case, since you aren't trying to share the
same antenna between the two radios.  But you also have the handicap that
most scanners are not designed with good selectivity or strong signal handling

If you want to make it work, I'd start by running your transmit antenna as far
in one direction as you can, and the receive antenna as far in the other direction.
Using directional antennas pointed at the repeater may help to minimize coupling
between the two, particularly if you can arrange the antennas to have pattern
nulls pointing at each other.  Run as low of a power level as you can.

My experience operating mobile with another car nearby is that my rig desenses
when I'm about 2 car lengths behind the other vehicle.  Often we do OK while
driving down the road, but lose each other at a stop light when we get closer
together.  So, at a guess, with standard mobile rigs you can probably get by
with about 50' separation between antennas, as long as there isn't any other
significant coupling between them.  This will also depend on the power level
and how strong the repeater is.

You can run some tests - monitor the repeater on the scanner while there is
another conversation going on, and try different power levels on your
transmitter to see how much power you can run before it affects reception.
(Some sort of attenuator is handy for this since most FM radios have a limited
number of output power options.)  Comparing those results to the required
power to hit the repeater will tell you how many dB more isolation you need
between the radios.

Posts: 1907

« Reply #3 on: July 13, 2012, 09:50:35 PM »

Since this isn't an absolute necessity, I'm looking for a cheap and simple solution.

This limits the range of possibilities. You can experiment with the antennas by using different polarization, i.e. one vertical one horizontal, and farthest possible distance. If you are talking about one particular repeater you could consider cavity filters. But now you are no longer talking cheap and simple.

Posts: 4

« Reply #4 on: July 16, 2012, 09:09:08 AM »

Thanks for the advise guys, looks like I may have to try a Yagi and and extra long headphone cord.

The cavity filters are something I hope wouldn't be brought up, seeing as how much they cost. But it kinda makes sense that my setup would have to be somewhat similar to that of a repeater.

We'll get back on the roof and experiment some more.



Posts: 2358

« Reply #5 on: July 16, 2012, 12:42:16 PM »

What about making a helical resonator Huh

Posts: 13017

« Reply #6 on: July 16, 2012, 04:49:42 PM »

You could try a helical resonator - you might get a Q of 1000 out of a well-built one.
That could give you about 150kHz at the 3dB points, perhaps 10dB attenuation
at the standard 600kHz spacing.   

That will help somewhat, but I think it will require significantly more attenuation than
you can get out of a single helical resonator.  At that point you might as well make
a cavity resonator out of a piece of copper pipe, some circuit board material, an old
fire extinguisher, or whatever else you can find.  The only difference is that the
center conductor is wound up in the helical resonator to save space - making it
close to a quarter wavelength with some end capacitance is a simpler construction
method and gives better Q if you have the space available.

But that still won't help much when the scanner has a plastic case - if it can pick up
enough signal straight through it without an antenna connected to cause desense
then nothing you add in the antenna lead is going to make any improvement.

That's why moving the TX antenna away from the scanner is an important first step.

Posts: 4366

« Reply #7 on: July 17, 2012, 04:01:24 AM »

I doubt that you'll get a loaded Q of 1000 out of a helical resonator. At least, not without a lot of attenuation.

Beer kegs are good for cavity resonators - but you need to empty the beer out first!

Posts: 306

« Reply #8 on: July 17, 2012, 07:12:00 AM »

Check with the repeater control operator, many have a feature that records about 5 or 6 seconds (maybe up to ten) and then plays it back to you over the repeater.  A good way to hear what you sound like over the air.

...bc  nr4c

Posts: 3476

« Reply #9 on: July 17, 2012, 04:07:32 PM »

Turn the power down on the HT and move away from the scanner.

Posts: 13017

« Reply #10 on: July 17, 2012, 09:26:40 PM »

Quote from: G3RZP

I doubt that you'll get a loaded Q of 1000 out of a helical resonator. At least, not without a lot of attenuation.

For that I'm relying on the nomograph on page 3.8 of my RSGB VHF-UHF Manual
(3rd edition, 1979).  It suggests that any helical resonator wound with a pitch of 6 tpi would
have an unloaded Q of 1000, and such a device would require a shield inner diameter of
about 1.6".  The same nomograph appears in my ITT Reference Data for Radio Engineers,
who attribute it to Macalpine and Schildknecht in the the Proceedings of the IRE, vol. 47,
no. 12, p. 2101;  December 1959.  An additional chart suggests that a 3" diameter helical
resonator would be capable of an unloaded Q of 2000 if losses were held to a minimum (which
the VHF-UHF Manual suggests includes silver plating the inside for operation above
about 100 MHz, though the original data are specified for seamless copper.)

That's not to say that a typical ham would achieve such a high Q, of course, but at
least it was considered theoretically possible some 35 years ago.

But the point is that, for the purposes of the OP, even a Q of 1000 wouldn't help a lot
in reducing the desense problem. 

Posts: 4366

« Reply #11 on: July 18, 2012, 03:00:21 AM »

The Qs listed are the unloaded Qs. As soon as you start loading the resonator (any resonator in fact)by coupling any appreciable energy out of it (i.e. lower attenuation), the Q goes down. Q is, by definition, energy stored divided by energy lost.

You get a double whammy because the antenna impedance is transformed up across the resonator, as well as the load impedance. The same applies in filters - see Geffe's 'Simplified Modern Filter design' or Zverev's 'Handbook of Filter Synthesis'.
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