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Author Topic: Stopping Inter-Mod Suggestions Please  (Read 717 times)

Posts: 0

« on: August 21, 2006, 07:47:48 AM »

Our club often operates several radio for events like Field Day, or more recently the Int'l Lighthouse Event. A typical set up is three staions; on 15M, 20M, and 40M. We are using bandpass filters for the correct band on each of the rigs. The coax feedlines for the antennas run parallel to each other out to the antennas. The antennas are usually fairly close to each other due to lack of good places to anchor to.

The problem: We still get intermod interference between the rigs. The bandpass filters help quite a bit, but 40M CW clobbers the 15M station (third harmonic) for example.

The question: Is there a way to eliminate or further reduce the interference?

Thanks es 73!

Posts: 719


« Reply #1 on: August 21, 2006, 08:02:43 AM »

There are two questions:  What kind of radios are you using?  What kind of bandpass filters are you using?

There are some radios that simply should not be used in multi-op environments.  My FT-840 comes to mind.  You should be using "contest" radios with high dynamic range and good filters.

Filters are quite variable;  the Dunestar and W3NQN filters are considerably better than the I.C.E. filters, although the I.C.E. filters do work well for many applications.  Stubs are cheap; but, go with real filters for your first level of filtering.

It is pretty unreasonable to expect to operate within a kHz or two of the harmonic on CW, more on phone, unless you have plenty of antenna spacing and excellent filters.

I'm not sure you're getting "intermod" as you describe it.  Intermod occurs when two narrowband signals are combined in a nonlinear device and you get modulation products at the sum and difference frequencies (and a few others).

Posts: 2527

« Reply #2 on: August 21, 2006, 08:31:26 AM »

I seem to remember www.K1TTT explaining how to make the coaxial traps he uses at his contest station.

Posts: 1421


« Reply #3 on: August 21, 2006, 08:50:16 AM »

A passive bandpass filter needs to see matched impedances at its input and output.  What was the actual impedance seen by each filter looking into the antenna feedline?

It may help to put a the filter between the transceiver and a tuner (or between the tranceiver and an amp) so that it is working into its rated load.

Posts: 564

« Reply #4 on: August 21, 2006, 09:30:26 AM »

Here's a link to the most pertinent of the K1TTT pages on stubs .There are more .The interfernce you describe is harmonic , not intermod .Since you're filtering already , you should probably look at differing antenna polarities , and distance separation , as much as possible .Even a little more separation can make a big difference .Good luck with your project .

Russ, W5RB

Posts: 9749


« Reply #5 on: August 21, 2006, 10:48:32 AM »

Stubs are cheap, not good.

The best filters will always be good L/C bandpass filters.

Sometimes a stub might help enough to make a difference, but they are generally useless for third hamonic problems.

I'd increase antenna seperation and look at better L/C filters. or maybe use a T netork tuner on the high band station and a pi network or parallel network tuner on the low band station.

73 Tom

Posts: 21764

« Reply #6 on: August 21, 2006, 11:18:09 AM »

At high powered multi/multi HF contest stations, we always used:

-Dunestar or equivalent, multisection L-C filters for each band;

-Directional antennas on separate towers, with as much spacing as possible both horizontally and vertically -- which is why most of these M/M contest stations that do well are on several acres of property;

-Chat mode in computer logging systems that are networked, so operators, besides making contacts and logging/duping them, can also "chat" with each other from station to station to say stuff like "NEW 1 WEAK 21.265 40M PLS SHUT UP A FEW" meaning will the 40m operator take a brief pause so the 15m op could hear the weak DX which would be a new multiplier.

All this stuff works.  But it takes setup and planning and space.

In contrast, we've successfully set up Field Day stations in just a few hours in ways that eliminate cross band interference completely!  Ways to do this:

-Use low power!  5W doesn't generate nearly the interference that 100W does, and can still make most of the same contacts.

-Use rigs that don't generate much out-of-band crap.  This is kind of a selection that must be made based on experience.  This is not typically "specified" anywhere, not even in the ARRL Lab reports.  It's just something you get to know with experience using a lot of different rigs.  A lot of the older analog rigs generate almost no noise off frequency, at all.  Some of the "earlier" synthesized rigs were terrible, by contrast.  A lot of the newer rigs are much better than the original versions.

-Separate antennas as much as possible, but you can also acheive electrical separation by criss-crossing dipoles, using small beams at different heights, making adjacent HF stations alternate from horizontal to vertical to horizontal polarization, etc.

I've set up FD stations that were as many as 25A (25 stations operating simultaneously from a single site, with 12 of those stations being HF phone and HF CW running simultaneously) with absolutely ZERO interference between stations.  And those stations set up in less than 12 hours.  But it takes some practice...and you can't do it "simply" if you're running kilowatts.


Posts: 0

« Reply #7 on: August 21, 2006, 12:31:04 PM »

We were using ic-718, ic-746, and ic-756 rigs all running 100W. The passive filters are MFJ (I think) little silver rectangular boxes if that helps. The antennas are very close, within 25 feet or so of each other and basically slopers off the lighthouse in the most recent event.  I also have a 500hz CW filter on the ic-718, but was still getting some hash when the 15M station xmitted SSB.  These are all standard stock rigs nothing fancy.  It sounds like the main problem is that our antennas are too close and on the same polarization which I kind of suspected. :-)  

Posts: 9930

« Reply #8 on: August 21, 2006, 05:17:01 PM »

I use an ice 409 filter to allow me touse several radios at once

Posts: 3289

« Reply #9 on: August 21, 2006, 09:01:49 PM »

I think your problem is not Intermod, nor Harmonic, rather pure Front End Overload.

With 3 HF antennas 25 feet apart, the sheer amount of rf energy is overwhelming the receivers.

Using the bandpass filters is excellent, but separating the antennas as much as possible is paramount. Orienting the harmonic antennas perpendicular to one another will help as well, e.g. 40M and 15M. Using a common ground point for all the radios has been suggested to me, but I have not tried it.

The other point made above is using the best quality radios with narrow filters installed. For example at last year's Field Day I used a 718 on 40M and it caused and received interference from most of the other radios. Multi-op environment requires quality equipment and setup.

Good luck! bill
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