AMATEUR RADIO REVIEWS
73 AMATEUR RADIO TODAY

73

Reproduced in full, as seen in the January 1996 issue of 73 Magazine.
For more information about 73 Magazine, call 1-800-274-7373.

Add-On 2 Meter Receiver Selectivity by Phil Clarke

With the proliferation of radio signal producers, both those that are supposed to radiate signals and those that do anyway, 2 meters has become a chore to listen to in many RF-dense areas. An RF-dense area can be a metropolitan area with loads of pagers,commercial repeater services, cable TV, and/or computers. It can also be a rural mountain, loaded with broadcasters and leased repeater services.

Types of Interference

Nearly everyone using 2 meters has experienced some form of interference at one time or another. Generally, the interference experienced will consist of:

Signal suppressing or blocking - caused by a strong RF signal which desensitizes the receiver, making it incapable of receiving the desired signal. Such signals are from transmitters on"near the 2 meter band" frequencies. A good example is the large number of paging systems located just above the two meter band.

Sidelobe interference - occurs when a signal on a nearby frequency (not within the receiver's passband) bleeds into the region of the passband and blocks the receiver.

Intermodulation - usually referred to as intermod or IMD, is caused by two or more signals from outside the receiver's passband mixing internally within the receiver, then blocking or otherwise interfering with the receiver's operation.

Using a filter such as the DCI-146-4H can reduce instances of these types of interference, often totally eliminating them.

The DCI-146-4H filter consists of four helical resonators mounted in a very solid chassis with two coaxial cable connectors. Helical resonators can be thought of as very high-Q RFcircuits, passing only a very narrow band of signals. In the case of this particular unit, that passband range is 144 to 148 MHz. The skirts are quite steep and provide up to -62dBm reduction in signals outside of the filter's passband. The DCI filter is passive and requires no outside power source. You receive and transmit through it with power levels up to 200 watts.

This type of filter (very narrow passband) allows the receiver to be undisturbed by most out-of-band signals, providing for increased sensitivity and clearer reception.

The Test

To give the DCI 2 Meter Bandpass Filter a real-world operational test, I took the unit to a nearby mountaintop where there is an abundance of commercial FM and TV broadcast transmitters and various commercial repeater systems (on all bands). In this particular area, the S-meter (which is an LED bar graph on my 2 meter transceiver) dances all over and, although the squelch is not broken at all times, the receiver is quite desensed. Even relatively strong stations cannot be heard well.
The test results on the mountain were quite positive. Only the slightest indication of intruding signals on the S-meter could be seen, and that was very sporadic. No noticeable degradation of signals was noticed, even when working distant repeaters and weak simplex stations. A quick removal of the filter gave instant proof that the filter really works, as without the filter the S-meter came alive and the distant stations were lost.

On the test bench I found that the unit's passband was a very close approximation to that indicated in the manufacturer's literature. Due to the very steep skirts of the DCI-146-4H, I doubt CAP and MARS users could use the filter. The SWR seen during transmit rises very quickly as you move out of band. During static testing I used a commercial dummy load and input power of 50 watts, although the filter is rated at 200 watts. Of note: No heating of the filter was observed during testing.

As a final operational test, I went near the transmitter site of a paging company that operates just above the 2 meter band. Using an HT connected to my mobile 2 meter antenna, I found the receiver to be blocked a large percentage of the time and the S-meter dancing a jig. I connected the DCI-146-4H into the antenna line and the problem ceased, with only a small amount of S- meter jumping noted. This type of setup should help the commuting ham using a handheld in RF crowded areas.

Comments

Installation is simple: Just put the DCI-146-4H in the antenna line by using a short jumper cable. The filter will do its job without any operator control - install and forget. Due to its size, you might wish to remotely mount the filter - just keep in mind that you want a reasonably short patch line from the rig to the filter. Firewall mounting appears to be practical on some vehicles (where there is space). Other choices are under a seat, in the trunk, etc. There are no mounting holes on the unit, and do not drill any!

My recommendation for mounting is:

1. Use plumber's strap over the unit and fasten the strap to the mounting surface with sheet metal screws.
2. Attach the unit to a surface with silicon glue.

The DCI 2 meter filter makes up for the wide-open front ends found on most 2 meter transceivers (wide-open meaning the receivers easily receive 135 through 170 MHz). When using the DCI-146-4H, effective receiver sensitivity outside the 2 meter band will be greatly reduced.

Too bad someone cannot design a filter that will remove all the computer hash heard around shopping centers, bands, and the like.

Availability

The DCI line of helical resonator filters is available at most ham radio stores and from DCI Digital Communications Inc. Note that DCI Digital Communications also produces filters for the 220 and 440 amateur bands. Commercial filters are available - call for specific applications.

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