RAM - INTERMOD ISSUES

Note, we were told by several people that they had difficulty in following the explanation of this issue. On 11/4/2007 we rewrote the web page, and hope this version is clearer. It is a technical issue that needs to be studied carefully to grasp the implications. ---RAM Staff.

Intermod. What is it? How does it affect us?

An intermod is an undesired mix of two or more frequencies.

Here in Nome, SPARC has met with an unfortunate situation.

The SPARC 2-meter repeater transmits 146.94 and receives 146.34. RAM, in turn uses 147.3 and 147.9. When both SPARC and RAM are on the air simultaneously, the following undersired mix takes place:

146.94   +   147.30   - 147.90   =   146.34

This mix is almost certainly taking place within the SPARC 146.34 receiver.

Note: no matter what channel pair RAM selects, it will affect SPARC. For instance, choose a frequency pair at random, like 146.52 and 147.12.

146.94   +   146.52   -   147.12   = 146.34    also.

This is because there is a relationship between the channel split, -0.60 in both systems. In other words, 146.94 - 0.60 = 146.34.

Note that the reverse situation also holds:

147.30 - 146.34 + 146.94 = 147.90 -- Then, why doesn't SPARC jam RAM?

The SPARC Nome repeater is not really a repeater unit as such. It is made up of two sites, one at the west end of town transmitting on 146.94, and the other at the east end receiving on 146.34. These two sites are connected by a UHF link which in turn is routed via a UHF repeater.

It was done this way because at the time it required no expensive duplexer/filter and it was a "quick fix" only meant to be temporary.

The radios in question are inexpensive, dual band Alinco or Yaesu units operating in crossband mode. There is no filtration or discrimination whatsoever in the receivers, and they are wide open to respond to a variety of signals arriving into them.

Note that SPARC can filter out any of the three frequencies getting to their receiver: RAM's pair, or their own '94 transmitter's. In fact, the reason the sites are a mile apart or more is that with closer proximity their receiver would be useless, so even without the RAM repeater in operation, it is a marginal proposition that just happens to have worked in the past.

RAM, on the other hand, uses a narrowband GE receiver, more robust, with sharply tuned helical resonator front end, and is further protected with a duplexer unit. For this reason, RAM does not experience the level of difficulty that SPARC does.

Indeed, if the RAM system were set up in Anchorage or any other big city with several ham repeaters, it would be dead on arrival.

Upgrading the SPARC equipment to meet good engineering and regulatory standards would alleviate the problem. As a matter of fact, outside of Nome many ham repeaters with the same frequency relationships operate even out of the same building and towers without problems.

Again, note that SPARC can filter out any of the three frequencies involved in the mix: They could filter out one of the two RAM frequencies, OR they can filter out their own transmitter from getting into their receiver. In effect, they are jamming themselves.

In case it is not clear, any repeater that is set up in Nome would also cause the same problem ... it is not RAM specific.

SPARC has, in stock, all the equipment needed to do it right. Upgrading the equipment is the better solution as it would also solve some gray area issues such as lack of remote control, etc. But a "quick fix" would be to change the frequency split. The 146.34 receiver could be changed to 146.24 or 144.64, and the problem should go away. Some years ago -but for different reasons- the odd-split repeater was ordered, but is in storage unused.

Update: SPARC conducted tests along with RAM on October 20, 2007. These tests were conducted by Nate, KL3NP with the assistance of WL7FZ, AL7X and KL0OH for RAM. RAM supplied various transmitting frequencies, while tried two different radios at the SPARC receiving site, as well as different receive frequencies.

It appears that the testing confirmed RAM's findings. Either radio responded to the intermod, and switching it from 146.34 to 146.24 (-0.7 split) resolved the issue. At the end of the tests, the radio was put back on 146.34.

SPARC is now evaluating their options. RAM will work with them to implement a solution satisfactory to all.

The following tests have been conducted by RAM

Subject to verification/duplication by SPARC:

• Transmission on any RAM frequency, or both, when SPARC '94 repeater is not transmitting do not produce any signal into the SPARC receiver or system.
• Transmission on the RAM repeater, 147.3 by means of the autopatch, so that there is no signal on 147.9, also produces no effect on SPARC when the '94 repeater is in use.
• The following arbitrary frequency pairs were tested while the SPARC '94 repeater was up:
  1. 147.12 and 146.52 Interference resulted
  2. 145.85 and 145.25 Interference resulted
  3. 146.55 and 145.90 Interference resulted
  • The following pair, which is NOT a 0.6 split:
    147.12 and 145.90 Interference did NOT result
• Update: We were reminded by several that this intermod problem also came up when the repeater was "on test" at KL2CT's house at the base of Anvil Mtn. Apparently, signal strength or distance is not an important issue, just the presence of the three signals.

Drive-by Tests

Transmitting 147.3 or 147.9 (individually, not simultaneously) did not produce any interference, along with many other possible channel frequencies tested.

The tests above are described here so SPARC technical staff can review them and decide for themselves as to the merits.

SPARC tests

On Oct 20, 2007 SPARC conducted tests of their own. The exact details are not known in time for this publication, but we will update as soon as we have the information. So far, this is what we understand:

• SPARC observed that their system would receive intermod when RAM and SPARC were in use at the same time.
• SPARC observed that their system received NO intermod when only RAM was active.
• SPARC keyed their repeater via autopatch, so no transmission was ocurring on 146.34. The same intermod occurred.
• SPARC swapped the Alinco 610 receiver unit with a Yaesu FT-8800R with no change in intermod.
• SPARC tested a different receive frequency, 146.24, and the intermod dissapeared.

When the testing was concluded, SPARC restored their receiver to 146.34. RAM concurs as it would take a while for the users to be advised, and more conferencing between the parties is needed.