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By Peter Policani K7PP

I know this is going to be an unpopular subject, but here it goes!

Let's look at other ways to head off the dreaded intermod problem.

In my last article, we talked about resonant cavities reducing the amount of unwanted signal coming down the coax from the antenna and mixing in the transmitter PA or receiver front end.

About 1965, a new device started appearing on the two way radio scene. It was called an Isolator. It was more than expensive, but it had a unique ability to reduce mixing of signals by allowing RF to pass by in only one direction. These devices started to be installed on many community sites where intermod was a fact of daily life. Sites were becoming crowded because the FCC had changed from wide band modulation to narrow band. (15 kHz verses 5 kHz plus or minus) Many two way radio users had gone to CTCSS or PL tones to try to keep their dispatchers from going crazy while listening to signals that didn't have anything to do with their operation. The big problem, of course, was the signals were still there. They were just masked by the tone coding.

Then it became a little confusing. Devices started to appear on the market call circulators. It looked as if the circulators came as one device with their 50 ohm loads as part of the unit. The isolators seemed to come with their 50 ohm loads external. They both do the same thing.

They allow a radio frequency within a band of frequencies to pass through the device in one direction. Any signal coming back the reverse direction, including VSWR, was routed into a dummy load and dissipated. Any signal found coming down the antenna, would be routed into another port and never see the Power amplifier of a transmitter. This means that an interfering signal would not have a place to mix. When added to a cavity, the effect is multiplied. The cavity restricts a narrow band of frequencies to those that may pass through the circulator. It is important to note, that a circulator is not effective across the entire band of frequencies and that if exposed to the whole spectrum, it may act as a non linear device on some portions of the band. In other words, never use a circulator without following it with a band pass cavity of some kind. (transmitter, circulator, bandpass cavity and then antenna).

Well, what happens when you use a circulator with a notch notch duplexer? (Remember, a notch duplexer only protects a receiver from it's own transmitter). Good point! Some frequencies sail right through into the circulator and cause it to saturate and become a non linear device.

Out of band signals can also find their way to the front end of your receiver. A wonderful mixer! YOU HAVE TO USE A BAND PASS SYSTEM!!!! A duplexer that has two ports on each of it's cavities will do the trick. If you don't have that type of duplexer, find a band pass cavity and put it in the transmitter leg after the circulator.

The same problem exists with the receiver's front end. Circulators don't usually find there way onto a receiver port. They would face the wrong direction. A good way to provide isolation from the outside world is to use a receiver with an aperture coupled preselector. These are very effective front ends and provide lots of isolation and protection from intermod. If you don't have that type of receiver, pick one up at a swap meet. Any old Motorola Motrac has one and will provide outstanding selectivity for about 5 dB insertion loss. You can scrap the receiver and just add the preselector to your front end.

5 dB loss!!!! Wow! Thats a lot of loss. What happens if you have 10 dB of receiver degradation? The addition of the preselector eliminates the 10 dB of degradation. Your net receiver improvement is 5 dB. Not a bad trade off. Many of todays modern ham units have very minimal preselection before the active amplifying device.(or should I say mixer).

Hams require sensitivities bordering on the ridicules in their mobiles and handhelds. You can obtain this kind of sensitivity if you eliminate effective preselection. Just having a .1 uv receiver doesn't mean it will work on a community site. How many times have you gone through the downtown area only to be inundated by paging tones and taxi cab dispatcher calls.

If you look at almost any brand of circulator, you will find that they supply about 35 dB of isolation and dual circulators 70 to 75 dB. Most site owners require 70 dB of isolation on all frequencies above 130 MHz. Ask your site owner about his requirements. Other bands like 6 meters, usually 35 dB. I am not sure about the reasoning behind this.

I haven't told you about the price yet. They are still expensive. A good dual port circulator can cost as much as a duplexer.

Again, I know this may be an unpopular subject, but I will bet that any amateur who has operated on a community site has run into a problem that has been caused by some one who is not running proper filtering equipment. In these cases, it's usually the commercial folks, only because there's more of them.

The communications field is still expanding and the FCC is going to split the channels again. This will mean double the existing channels now in use. You can bet the community sites will continue to be loaded with new equipment and much of it will be in the 220 and 900 bands. You can also bet we will see a resurgence of UHF equipment, this time it will be trunked and more than likely digital audio. I don't know how easy it will be to identify an interfering signal from just bursts of data.

I know that adding a piece of expensive equipment when It doesn't do anything for your range or power is a pretty tough pill to swallow, but it can make your equipment hear better, but only if everyone uses them.

When is a site big enough to require mandatory circulators and cavities?

Good question.

It's going to be up to the site owner to say when, but thats how sites become saturated with intermod problems. Stations are added one at a time and one day, you have dozen's and it seems as if it happened overnight.

Mandatory filters? I don't have a good answer. Most locations, though, are already developed radio sites and the answer is simple. I know that many of you are going to read this and then file it, and thats OK, but file it in a place where you can lay your hands on it. If you haven't been asked to equip you station with the proper IM equipment, it's just a matter of time.

The following is a list of part numbers of circulators that I have been using. I don't endorse the brand, it's just one I happen to use.

  • DB products
    • Single stage isolators
      • All figures are 35 dB isolation. All models will handle 125 watts.
        • DB4612-1A 2 meters
        • DB4631-1A UHF
    • Dual stage isolator.
      • All figures are 75 dB isolation. All models will handle 125 watts
        • DB4612-2A 2 meters
        • DB4631-2A UHF
          • Maximum bandwidth is 4 MHz
Pete Policani, K7PP

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