Repeater desense?

Discussion in 'Ham Radio Discussions' started by WA7ZAF, Apr 18, 2019.

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  1. WA7ZAF

    WA7ZAF Ham Member QRZ Page

    I am setting up a repeater on 224.34 in my community. I bought the Bridgecom new a little over a month ago. While waiting for my duplexer to arrive, I have been tinkering with it using vertical separation between a Hustler G7 (9dBi) and an inverted 1/4 wave ground plane. After talking to two hams who have claimed to have successfully run their machines with less than 15 feet between antennas, I decided to settle on mounting the G7 at 27 feet with the 1/4 wave located 14 feet below it. That's the best I can do right now.

    I drove around town with a couple of handhelds to check my coverage and was disappointed in my real world results as compared to what I had mapped with software. At times, when I listened to the repeater coming back, all I could hear was a rhythmic clicking sound. I just assumed that I had reached the limit of the range of the machine's coverage. I guesstimate that the real world coverage was maybe 1/3 of what I saw in the software.

    Finally, I did some testing with a friend who lives about ten miles away with some challenging terrain between us. On an external, 1/4 wave, aerial antenna, we can simplex on 5w, sometimes less, with some static on our signals. When I have the repeater transmitting into the low (1/4 wave) antenna, he can hear me well with some steam on the (20w) signal. When he tries to get into the 9.15dBi G7 receive antenna (25w on Alinco 235), all I hear is the same clicking sound from my testing experience. When I switch antennas, he can hear me "booming in" on the G7. He tries to hit the repeater and I either hear the clicking, or nothing at all, not even breaking squelch.

    I'm guessing that the desense from the antennas not having sufficient vertical separation is the problem. Does this sound accurate? If so, why can I do well at short distance with a 5w ht? This is my first repeater and I'm just trying to wrap my head around this.
  2. K8JD

    K8JD Ham Member QRZ Page

    Were you transmitting on one hand held and listening on another :eek: , right next to each other ??? that's the problem, the transmitter in one handheld was deseinsing the other one so it could not receive !! THAT NEVER WORKS :D
    Sounds like you still have a big desense problem at the repeater also !.
    I worked on commercial repeater systems for 43 years, Been there-done that !~
    You need a helper to stay behind while you drive around to report back to you.
    The split antennas thing only works with like 50 ft of vertical separation on a 300 ft tower !
    Wait for the duplexer to arrive.
    K4AGO and KX4O like this.
  3. K7JEM

    K7JEM Ham Member QRZ Page

    Yes, most likely the case. In order to understand what is going on, you need to realize that there is a certain minimum number of dB of isolation required for a repeater to operate without desense. That number will depend on specifics, like TX power out, RX sensitivity, RX overload tolerance, and TX phase noise. There are other things that can contribute as well, but these are the big things. I don't know anything about that specific repeater, but we can throw some typical numbers around and get into the ballpark.

    14 feet of vertical separation is going to get you around 48dB of isolation at 224 MHz, which right off I know will not be enough. If we calculate the antennas as if they were two dipoles, and figure that the TX is running 25 watts of power, we can see that the power transmitted will be close to 20 watts, after factoring in the feedline loss. That is equivalent to +43dBm (43 dB greater than a milliwatt). That means the power arriving at the RX will be 48dB less than that, or around -5dBm. Now, -5dBm is a pretty hefty signal to deal with by any RX, and will likely cause a severe amount of overload. That won't damage the RX, but it will desense it to the point that only really strong signals will get through. The clicking, or on-off cycling, is coming from the fact that the signal is not initially desensed, while the TX is initially off. So the RX hears the weaker signal, turns the TX on, then the RX signal is immediately desensed, and the TX runs for the normal carrier delay, then drops out. Once the TX drops out, the RX again hears the signal, and keys the TX, which again wipes out the RX until the carrier delay times out. This process repeats until the input signal goes away.

    The duplexer, or antenna separation, are the ways we have to isolate the TX from the RX. Typically, we would need to see around 75-80dB of isolation to get good results on a repeater like this. This would require 50 feet of vertical separation, more or less, depending on antennas and tower structure, etc.

    The other side of the desense issue is determined by the phase noise, or wideband noise present from every TX, even somewhat removed from the TX frequency. This number is a big unknown, because it is different with almost every TX made. Some are very clean, others not so much. But lets say the noise is down 100dB, relative the carrier, at the RX frequency. Again, we know the power out is +43dBm and the isolation is 48dB. We start then at -5dBm, if all of the carrier was "on channel" (as calculated above). Then we know that the noise will be 100dB below that level, or at -105dBm at the RX channel. That's a pretty weak signal, but your RX is likely to be able to open squelch at -120dBm or so. That means your TX is 15dB stronger in noise than the weakest signal your RX is capable of hearing. That noise will desense out all of the weaker signals. To get through the repeater, a signal would have to be around 6dB stronger than that noise, so lets just say the signal would need to be -100dBm to keep the numbers round. A -100 dBm signal is 2.24uV, which would normally be fairly strong, nearly full quieting, but because of the desense might just barely get into the repeater (or not).

    It would probably be possible to use 2 antennas with 14 feet of separation, if each leg had a single pass/notch cavity in line. A single cavity of this type will provide around 35 dB of additional isolation, which would bring your total isolation up to 83dB, which normally would be enough isolation to work well.

    All of these numbers are hypothetical, but probably pretty close. It is easy to see that you could be experiencing desense based on RX overload, or TX noise, or both. Probably both.
    K0UO and K4AGO like this.
  4. WA7ZAF

    WA7ZAF Ham Member QRZ Page

    Thanks for the replies. They confirm my understanding of what is going on. I don't like to be too arrogant when I have these conversations with my local ham friends because they have more experience than I do, but I keep running into bad advice. This time one keeps telling me that I have bad feedline, and that he knows a 220 repeater will run fine with the amount of separation that I have. My research shows differently. He gets frustrated with me when I disagree with him, but I get frustrated too when he treats me like I know nothing just because he has more experience in the hobby than I do.
    K4AGO likes this.
  5. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    There is a big difference between those with, like, 10-years experience and those with 10 1-years experiences! Both persons have been licensed for 10-years but there is a BIG difference in what each person has actually learned!

    Having owned commercial community repeaters and also having owned amateur radio repeaters, there are a lot of factors that enter into the equation of getting optimum performance from the repeater.

    Also, remember that the life of an amateur radio repeater owner is just like the life of a small boat or small airplane owner. The 2-happiest days of their life is the day the repeater goes on the air and the day that the person no longer owns the repeater. The in-between days are going to be from barely livable to absolute Hell! There are going to be users who complain about every facet of the operation. If the repeater "burps", no matter what time of day or night, you will get telephone calls complaining about the "burping". Some users will complain about the coverage of the repeater while others will complain about still other users. The persons complaining the most will, generally, be those who contribute nothing to the operation of the repeater be it technical or financial. In addition, the financial drain of operating the repeater will "add up"!

    I have never owned a small boat. However, I have owned amateur radio repeaters and a small airplane (Cessna 150). Both can be headaches that are best relieved by getting rid of the object!

    Glen, K9STH
    NL7W and K9ASE like this.
  6. AJ4GQ

    AJ4GQ Ham Member QRZ Page

    Your friend reminds me of an ex-employee. He had over 20 years of experience but performed like he only had 3 months of knowledge.
  7. K7JEM

    K7JEM Ham Member QRZ Page

    A 220 repeater "could" work with that amount of separation. However, it would be a RX with a very sharp helical front end, and it would need to be a TX with extremely low phase noise, and probably relatively low power output.

    On the 440 band it is much easier, especially using a commercial radio like a Micor. There, you have 5MHz of split from TX to RX, and the helical resonator can deliver an additional 50dB or so of isolation at 5MHz. Also, the TX is very clean on a Micor, since it uses a crystal oscillator and tuned circuits in the TX chain. I have seen them work fine with 10' of vertical separation, running 25 watts or less of power.

    But most modern repeaters use synthesizers that are much noisier than a crystal. And most of them do not have sharp helical resonators in the front end, such that virtually all of the TX power at the RX antenna ends up at the first mixer, which only creates problems. A rule of thumb is that you want your TX energy to present a level of -30dBm or less at the RX mixer. Your antenna isolation, helicals, and duplexer or cavities need to provide that isolation.
    NL7W likes this.
  8. WB2WIK

    WB2WIK Premium Subscriber Platinum Subscriber QRZ Page

    Having built, owned and operated VHF repeaters since 1974 (my first was WB2WIK/R on 146.805 MHz in NJ) in my experience "everything" causes desense.:)

    I've never been able to get away with aluminum antennas having clamped connections, or anything less than hardline/Heliax as the transmission line, or anything less than double-shielded coax as duplexer cavity interconnections, and on and on. At least not on 2 meters. On 440 MHz, things are a bit less critical due to the wide frequency separation. 222 MHz is in-between.

    The only times I've actually gotten a 2m repeater to work with two antennas and no duplexer was when the antennas were very widely separated, like "miles." We did this for about a year on the 146.985 MHz "Splitrock" repeater back in the early 1970s...the TX antenna was at the final repeater hilltop site, but the receiving antenna was another Stationmaster at about 70' above ground at someone's house (about 2 miles away) and the link was via a dedicated telephone line. No desense, but of course TX and RX coverage were not the same. For locals within 5-10 miles, coverage was fine, but going over the horizon -- not so much.

    The G7-220 is a very good antenna but it has several clamped connections that could cause noise issues.
    NL7W likes this.
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    Every time I have used a split antenna system, I have used a very sharp filter on the receiver.

    A sharp filter with a pre amp is easy to build compared to a transmitter filter.
  10. K4AGO

    K4AGO Ham Member QRZ Page


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