PI2HVN relais station Heerenveen

Around the year 2000 I joined the crew of relay stations Heerenveen. After a few years of writing software for mobile radios, relay stations are much more interesting because of all other aspects around the radio hobby. All repeaters from Heerenveen at that point were retuned Storno radio’s, these have worked fine for years.

Later we decided to use the nice looking RS9044 because of its specifications, shortly after the RS9044 we replaced the transceiver by a diversity system of the same brand. The RS9045 is for 90 percent the same as the RS9044 but with one more receiver, an extra IF circuit and a noise detection circuit to determine the best signal. Since both receivers are identical, fast switching between receivers on the fly will not be heard.

Output frequency: 430.025MHz

Input frequency: 431.625MHz

CTCSS controlled: 82.5Hz (under normal circumstances)

QTH: JO32WW

Repeater is carrier operated, but when using CTCSS the squelch will be opened even if your signal is very weak. At the moment the CTCSS option is disabled because of merging PI2HVN with the Coversity project.

After replacing the Storno radio’s we had one major challange: a spurious signal which appears at the input frequency as soon the transmitter is on air, causing the station to stay on air and oscillate.

In some circumstances, depending on the weather, the relay station kept itself on-air once the transmitter was switched on by a user. A loud oscillating sound was heard and somebody had to climb the building again to shut it off. This issues lasted for years, a few weekends leaving the spectrum analyser at the location logging the spurious, replacing cables and antenna, replacing transceivers... Everytime we thought we had solved this issues, it came back withing a few weeks.

The goal is to make the receiver as sensitive as possible, this is good for a faw away station but unfortunately also for a very weak annoying signal. The spectrum analyser is not that sensitive by far, it sees a signal at -90dB while the receiver is as good as -140dB.

The problems started around the same period we replaced the Storno, so at a certain point there was a doubt about the Radiosystem. To exclude spurious from the PA, the PA was replaced by a completely different amplifier. No result, still oscillating from time to time.

To make the confusion even bigger, the disturbing signal appears at a few particular transmitter power levels, but also depending on temperature, moisture etc.. It seemed reproducable at 13W most of the time, but sometimes it appears only at 8W or 15W output power.

Next experiment was using two different mobile radio’s, one for transmitting and the other for receiving the same time. This was more difficult because the receiver of the mobile radio is not that selective as the receiver of the RS9044, the strong signal of the transmitter could be a problem for the mobile receiver. Fortunately, the disturbing signal showed up at a very low power as well! The mobile receiver did also receive the signal at the reception frequency of the relay station, the proof was there: It is not our equipement but from the outside!

To be sure about the Radiosystem, we went to a place far away from any possible interfering signal.

After doing our very best with quite high power, manipulating the position of the transmitter and receiver antenna, a church song appeard out of our relay. The transmitter frequency at that moment was 430.875MHz, and the receiver at the original 431.625MHz. The difference was 750kHz, close to a radio station in the Netherlands at 747kHz which was indeed in the middle of a church program that moment. Okay, when stressing the radio’s front-end it is always possible to create intermodulation.

Then, at the location of the station it could be passive intermodulation. The whole building where the station is located is covered with metal. Any oxidation between the nails in the wall could cause a diode effect and generate unwanted signals. So while the relay was oscillating, we walked around the building and pushed the walls and antenna hoping the oscillation stops or at least changes. It didn’t.

At a certain moment i put my handheld transceiver on top of the central heating system, just didn’t want to keep it in my hands all the time. When searching for the disturbing signal -again- by changing the transmitters output power it turned out the even the handheld was hearing the signal. This was a breakthrough, just accidental and a bit lucky that the handheld was not disturbed by the transmitter output of 10W at that point. The handheld was a Kenwood TH-D7E, two other Icom type were not able to hear the weak signal close to the 10W transmitter signal 1.6MHz away.

Continuing the experiments with the Kenwood, varying the output power seemed to let our disturbing signal drift away. Some notes, just in case we might get a clue.

430.025 is our transmitter frequency, okay the disturbing signal is around that frequency... Great but where does it come from?

There were locations inside the room, where the relay station and the central heating for the apartments below, was as strong as an S9 signal. Should be traceable... That was a reason to ask an enthousiastic radio directon finder. He was equipped with an 2 meter ARDF device and a down converter to receive at the 70cm band. A big advantage of an ARDF device is the adjustable RF gain, when searching close to the signal it is very useful to reduce the sensitivity instead of trying to head the difference between maximum and maximum.

At the location of the relay it was hard to find the direction, the 10W transmitter signal slightly affects the receiver. Ofcourse pointing the antenna into another direction also changes the level of the transmitter’s signal which enters the ARDF receiver.

At first sight it seemed to come from the citycenter, so we continued searching downstairs heading towards the busy city center.

There was no obvious signal apart from a weather station 300m away and a computer system at the corner. The signal at the corner seemed to be a computer, fortunately people are usually willing to help. Only in a few cases we have to ring and explain some things but in most cases people are asking what’s happening when two radio amateurs are lingering around the house with a huge antenna and noisy handhelds. Ofcourse, it looks funny.

The computer was not the problem, it produced a strong signal at the input frequency 431.625 MHz but was not heard by the relay station.

Finally, back upstairs at the relay station again, there was still the signal at the input frequency but it looked like reflection against the wall of the apartment. Hard to find out. However, in the staircase the signal seemed to come from one level below us... Searching two levels lower the signal came from upstairs again. Would it at last be the neighbour below the station? Fortunately somebody came home at that address, we were allowed to come in and the signal went stronger and stronger... Finally, closer and closer and at a certain moment i had to remove the antenne form my handheld, the signal was too strong to see any difference on the signal meter. We were pointed somewhere behind the tele, after switching off equipment one by one it turns out to be a receiver of a remote controlled power point.

After chatting a while it tuned out that this neighbour moved here in the same period as we changed from Storno to RS9044. Suddenly everything was clear now.

A few days later i went to the DIY shop to buy a new set to swap them, but just to be sure i did some measurements. To my astonishment these brand new devices are producing huge spurious as soon a signal around 430MHz appears. Does not matter if it is 30mW or 50W, there is noise produced between 380MHz and 460MHz. Taking a closer look at the spurious generated by the receivers:

The largest signal is the repeater output, all others are spurious signals produced by the receiver.

Most of these remote controlled things are working on the 432MHz LPD frequency. After breaking open one of these receivers it seems to be very cheap designed, just a super regenerative receiver which resonates at 432MHz. When a signal close to the resonant frequency is applied the circuit this spurious is generated, when the power of the relay is increased, the shift between the spikes is incrreasing. The amplitude is about the same but the space between the spikes is changing. If the relay station is receiving 1.6MHz higher, there is a great change that one of these spikes appears at the receiver frequency.

I have a number of different brands of these receivers tested but they all behave the same. This kind of design ofcourse is very suitable for such an application, very cheap and no issue to detect a wide modulated FM signal. So the most safe solution was to look for an 862MHz version, we have found one which was 6 times the price of the 432MHz type but after getting in contact with the station’s neighbour to swap then, the problem was solved.

These remote controlled sockets are very popular, in the Coversity project there are a number of repeaters involved and now ans then the typical sound of these sockets is heard. I think this is an important remark for repeater builders.