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Author Topic: spot frequency blocking required... best appraoch?  (Read 4891 times)

Posts: 52


« on: July 25, 2003, 03:04:22 AM »


I operate a 25w beacon on 28.209MHz (EI0TEN). Unfortunately, I've recently had to relocate the beacon to my home qth and here it'll remain for the foreseeable. Consequently my ability to operate on 10m (and to a lesser extent 12, 15, 17 and 20m) is significantly compromised.

Any suggestions for how I can best filter out the 28.209MHz transmissions in the rest of my station? I've looked at Dunstar filters etc. which are fine for the bands other than 10m. What can I do about 10m however? Is a very narrrow bandstop filter a possibility (i.e. built to stop 28.209MHz)?

Any advice and/or suggestions appreciated.

Cormac, EI4HQ

Posts: 229

« Reply #1 on: July 26, 2003, 12:46:14 AM »


This sounds as if you will need a crystal filter ahead of your receiver, configured not to pass a narrow bandwidth, but to _block_ it. The good news, as they say, is that it seems possible. The bad news is, very few people (if any) have ever DONE it. Motorola did have crystal *bandpass* filters for VHF (I've got one in the junkbox, somewhere) but I've never seen a band-reject crystal filter ahead of a receiver.

You might get some good ideas looking at the old-time crystal IF filter circuits, which could be adjusted to place the notch where it was needed.   Naturally, you should not try to transmit through such a filter.

Good luck!



Posts: 173

« Reply #2 on: July 28, 2003, 01:31:48 PM »

Depends on what you want to do.

The first thing you need to take care of is to make sure that your beacon transmitter is ultra-clean.  If that transmitter is based on one of the several ten meter monoband rigs that have been popular in the past 20 years, the transmitter is dirty in a couple of ways.

Any frequency that is generated internally is a part of the output spectrum.  True, they are attenuated 60dB or more, to meet FCC and other agency guidelines.  However 60dB down from 25 watts is still 25 microwatts - which is a LOT of signal into your receiver.  Every one of those signals carries with it some phase noise, and therefore they bleed rather widely.  If you are hearing your beacon on all frequencies as "keyed hiss", it's phase noise you're hearing.

To combat the out-of-band stuff, you'll want a 10 meter bandpass right on the transmitter.  There have been bandpass filter designs in the ARRL handbooks for years.  Build one, and have a ham with some test equipment sweep it for you and tune it as close to your beacon frequency as you can.  In this case, you're not going to care about how narrow the filter is on 10 meters, you want maximum rejection on everything else.  This filter goes between your beacon transmitter and your beacon antenna.

Along the same topic, of keeping the beacon rig clean, it should, of course, operate on its own power supply.  12 volt batteries float-charged will isolate it from the AC line the best.  Whatever is the connection between your AC line and the power supply for the beacon rig should have a very good line filter on it, intended to block the 10 meter signal.

The third bit of filtering for the beacon rig should be a helical resonator filter.  This is a filter that is very narrowband, and will be centered on your 10 meter beacon frequency.  The stopband rejection isn't great on other bands, that's why you still need the first filter I mentioned.  You can read up on how to design helical resonator filters from the 1999 ARRL handbook and probably other editions.

Also, make sure that your 10 meter beacon transmitter is radiating ONLY from the antenna.   Put ferrite beads (the snap-on variety often works well) on the antenna's coax.  Speaking of coax, this might be a good place to use a better-shielded coax.  RG-214 is a nice double-shield coax.  RG-6 is a 100% aluminum foil shielded coax, it's 75 ohms, but that mismatch doesn't lose much power - or you can use a cheap antenna tuner at the rig to tune out the mismatch.  Amateur grade RG-8 is not a great choice here because the shield coverage is only 97-98 percent.

So now you've done your best to clean up the 10 meter transmitter - that's where most of the work was.

On your receive side of things, you can also consider an AC line filter for your power supplies.  I would hope you don't need to double-shield the coax on your main station, but using lots of ferrite beads will help anyway.

A notch filter, tuned right at your beacon's frequency, is going to be imperative.  But will it do everything you need it to?  A very simple and inexpensive filter can be made from parallel-wire transmission line.  The higher the line's impedance, the narrower the filter, and the more loss!  You can easily get 450 ohm ladder line.  One quarter wavelength long, open circuit at the far end, will be a pretty deep notch, probably 30-40dB.  You place one end of the line in parallel with your incoming receive antenna.  One leg to ground, one to hot.  To achieve the 1:1 balun function you might need, you can put ferrite beads right over the ladder line - they'll be sort of big ones, of course.  They must go around BOTH conductors.  An individual ferrite bead around each conductor will make the line a low-pass filter which you don't want.  I've seen a few hams use a piece of small 300 ohm TV twinlead with ferrites for the balun.  As much as 10cm of such line, operating on ten meters is too short to cause a major impedance lump.  Just solder the "far end" to the ladder line.  That way you can use normal-sized beads.
You tune the t-line filter by trimming the length.  Start with it too long, and find out where it notches best.  It'll be about 100kHz wide, I think.  Best way is to create a broadband noise source in your vicinity - American made light dimmers do a great job of making broadband noise - and finding out by listening on your receiver where the noise is least.  Should be lower than your intended frequency.  Try trimming about 3-5 cm from the line and see how far the "notch" has moved.  Keep doing this until the "notch" is where you want it.  The transmission line filter WILL detune if you roll up the cable.  Best if you can lay it out straight.  You're only going to have some two meters of it anyway.  I do not recommend this "stub" to be inside your shack because it will do some radiating when you transmit, and if you operate with power, you don't want that much radiation of yourself.

If this notch filter works effectively to get rid of the interference you're getting from your 10 meter transmitter, then you have hit upon a good method to solve the problem.

Problem is, you can't hear much on 10 meters now because that notch filter is working so well.  Now is the time to embark upon the design of a crystal filter - that can create the notch as well as the transmission line, and be far narrower.

It may also be possible to make the notch with a helical resonator, but it won't be as narrow as the crystal, although you can make them more narrow than a simple transmission line notch.

Also, the transmission line notch filter will cause poor SWR if you transmit with it inline.  A tuner will help with that, but better to take it out of line during transmit.

This is all a lot easier if you don't plan to operate on 10 meters.  But I doubt that you'd be operating a 10 meter beacon if you didn't enjoy operating the band!

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