Getting the most from your repeater system

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Back to Home Getting the Most From
Your Repeater System
By Kevin Custer W3KKC
HTML'd, edited and maintained by Mike Morris WA6ILQ

First, keep in mind that a repeater's performance is usually limited by its ability to hear the users, not by its power output. Minimizing the attenuation of the received signal by improved receiver performance, minimizing feedline loss, optimal duplexer tuning, etc. is one of the least expensive ways to maximize the useful range of the system. Once the incoming signal is down to the site's noise floor, there is nothing you can do to get it back. To reiterate, if you can't hear the users then the repeater is just that much useless hardware sitting on a hilltop.

Tuning the Receiver and Transmitter
A good starting place for getting the most out of your repeater, is to verify that your transmitter and receiver is working up to snuff. This may seem a difficult task, especially if you don't have a lot of test equipment. Transmitter alignment is essential for good duplex operation. Follow the alignment procedure for the exciter, then tune the amplifier for maximum on-channel output using a good quality watt meter and dummy load. Use a weak on-frequency signal to tune the receiver. This is normally accomplished with a Service Monitor, however you may not own one of these.

You may want to check around and find a ham that works for a local two-way shop to see if he will assist you in tuning up your receiver. If a monitor is not available, use a signal from someone at a distance from you with a stable operating station. Someone with an all mode rig with variable output power is a big help. I've seen the local oscillator leakage of a Bearcat scanner sitting 25 feet and later 50 feet from the repeater receiver used as a source of a test signal. Tune the receiver without the transmitter keyed. If your repeater doesn't have a PTT disable switch, add one, a simple toggle switch is fine. The switch will allow you to easily turn the transmitter off and on to test for desense. The most useful configuration is a center-off toggle switch with the wiper hooked to the transmitter's PTT lead, one side hooked to the repeater-controller's PTT output, and the other side grounded. In the center position the transmitter is disabled, in the up position it's keyed full time, and in the down postion it's normal (keyed by the repeater controller).

Duplexer Tuning
After verifying proper transmitter and receiver operation, connect the receiver to the duplexer, let the transmitter on the dummy load for now. Place (another) terminating load on the transmitter port of the duplexer. The terminating load can be as simple as a pair of 100 ohm resistors in parallel across the proper connector. Connect a weak signal to the antenna side of the duplexer, with either a service monitor, or from the antenna with your weak signaled friend. Tune the receive pass section of the duplexer for maximum quieting while watching a signal strenth metering point (called the 1st limiter in some manuals). Alternatively you can use a VOM hooked to the local speaker leads, look in the receiver manual under the heading "20 dB quieting test" or "SINAD test"). You may need to optimize the input tuning stage of the receiver for maximum quieting. Go back and forth between duplexer and receiver, tuning each for maximum quieting until no further improvement is measured.

With the transmitter still connected to the dummy load, and receiver still hearing a weak, noisy signal, you should be able to key and unkey the transmitter with no change in receiver quieting. This step tests for internal repeater desense. If this test fails, the repeater itself needs work to get rid of all internal desense. If all is OK so far, remove the load from the transmitter port and connect a handheld, tuned to the repeater output frequency, to the duplexer transmit connection. Connect the repeater antenna through a quality thruline wattmeter to the duplexer antenna connection. Key the handheld and tune the transmit pass section of the duplexer for maximum output power as indicated on the wattmeter. It is important to start with low power, 1 to 2 watts. If the duplexer tuning is not close to resonance you may damage the duplexer, or the handheld.

Disconnect the handheld and replace it with the repeater transmitter. Tune the duplexer transmit pass section for maximum output. If you are tuning a tube type transmitter, tune the duplexer for minimum reflected power, (the wattmeter connected between duplexer and the transmitter), as the tuning of the duplexer will change the reactance of the output section of the transmitter, and upset the tuning and loading of the transmitter.

Now, connect the signal generator to an outside antenna capable of opening the squelch on the receiver. With the transmitter disabled, turn on and adjust the weak signal source for about 50% to 75% of white noise. Turn the transmitter off and on, do you hear any difference? If you do, you still have desense, and the notch section of your duplexer need tuning. If the signal is completely wiped out, you have a lot of desense. In order to tune the notch side of the duplexers the receiver must hear a weak signal. Increase the weak signal if necessary to maintain open squelch with some quieting. While listening to local speaker and transmitter keyed, adjust notch section of transmit side of the duplexer for maximum quieting. Reduce weak signal level if necessary to maintain a similar level white noise. If the receiver becomes quiet while adjusting duplexer, you won't be able to tell if you are making an improvement. Reducing the signal will allow more critical adjustment. Keep adjusting notch section of transmit side of the duplexer and reducing signal level until no further improvement. Do the same for the receive notch section of the duplexer. You should have eliminated all desense.

Always strive for NO measurable desense if at all possible (and use test equipment, not your ear). The use of a simple toggle switch in the PTT line, and (as mentioned above) monitoring the noise level of the receiver on a VOM hooked to the local speaker leads is one good way to test for desense. Use a weak signal leaking into the Antenna System, not by inserting it into the receiver, or duplexer line with a "tee" connector or some other unacceptable manner, (everyone else has to go through the antenna, your test signal should also). I use an outside mounted test antenna - something like a 1/4 wave ground plane or another antenna already mounted on the tower works well. It is necessary that the antenna be fixed in position and away from your body so the received signal level does not change as a result of of you (or anyone else in the building with you) moving around near the test equipment or repeater.

If you still have desense, try connecting a dummy load in place of the antenna because some antennas just don't duplex well. The test antenna should be several wavelengths above the repeater transmitter and receiver. This vertical separation between antenna and radio set keeps RF from bombarding into the circuits of the repeater. If the repeater and duplexer can be tuned into a dummy load with no desense, your antenna, feedline or lightning aresting equipment may be defective or may not duplex well.

A note from WA6ILQ:
The most common cause for "desense" is connections, and not the duplexer. This is especially true if there was no "desense" prior. I would first check the antenna, the jumper cable between the antenna and the Heliax feedline, the feedline itself, and then the the jumper cable between the feedline and the duplexer. Start by placing a ThruLine meter right where the feedline comes into the building. If you get any reflected power, then your desense problem is most likely between the antenna and the meter. Then repeat the test between the duplexer and If that looks good, then check all the connections on the station (before you start turning the screws). I have had to re-tune many a duplexer that someone turned the screws on because of "desense" that turned out to be a cabling issue and not the duplexer. Good quality equipment like name brand duplexers and quality feedline do not change settings by themselves - something else in the environment does.
One nasty issue was the LMR series feedline... it is made with an the outer jacket over copper braid over an aluminum foil shield. Moisture creeping into the cable can oxidise the aluminum and the aluminum oxide becomes a source of desense.... slowly increaing until it exceeds the strength of the incoming users signal. The only solution is to replace the entire run of LMR cable with good cable.
Note that a few of the LMR-nnn types have a mylar barrier between the foil and the shield mostly eliminating the dissimilar metal problems. So before you buy it check the construction.

Cabling Concerns - (in-cabinet)
Never use 9913, LMR-nnn or similar foil shielded cables as in-cabinet jumpers in duplex service as they will generate noise. A few of the LMR-nnn types are fine as they have a mylar barrier between the foil and the shield eliminating the dissimilar metal problems of 9913 water hose. Andrew 1/4" Superflex is the preferred in-cabinet interconnecting cables (watch the minimum bend radius) but expensive. RG-400 (NOT LMR-400 !!!) is the size of RG-58 and is double shielded silver plated flexible coax and is one of the better interconnecting cables to use, another is RG-214 (size of RG-8) but it has a silver-plated steel center conductor and can't take too much flexing (again, watch the bend radius). Do not use RG-142 if you can avoid it (see this web page for the reason). Your goal is 100% shielding and common HF-grade coax will not provide enough shielding for proper receiver-to-transmitter isolation. If your existing system is having problems then swapping out all of the RG-8 / RG-8X / RG-58 / RG-213 / whatever jumpers with RG-400 and using the proper silver plated connector at each end (rather than an adapter or worse, several stacked adapters) can make a huge difference in the RX performance. If you absolutely HAVE to use an adapter, always use a silver plated adapter (Kings and Amphenol are the preferred brands), never chrome or nickel. And make sure you replace that adapted cable with the proper one on the next trip to the site.

Go to the "Antenna Systems" page at this web site and read the section on "System Engineering" and on "Feedline, Connectors, Coax and Shielding".

Cabling Concerns - (cabinet to antenna)
As far as the link from the cabinet to the antenna, you want to use something with a 100% shield. Never, ever use a cable that has dis-similar metals in contact (like 9913 & LMR-400 or LMR-any-number for that matter). Nothing beats Andrew Heliax (or a similar corrugated copper line). Period. End of discussion. (And a lot of commercial sites won't allow anything else).

Cabling Lengths
Most times you simply cut the cables to a convenient length, put connectors on them and they will work fine. The only time they need to be specific lengths is when there is a wavelength issue involved (like the inter-cavity jumpers on a duplexer, or the inter-bay harness on a multi-bay antenna), or when something isn't quite right. There are instances where cabling lengths will be critical between duplexer and the radio set. Usually when this is a problem you don't have the correct impedance matching (even when the pass sections are peaked) and it's likely the SWR will be high back on the repeater transmitter and / or it will make the transmitter fold back the power if it has VSWR protection. You can augment the length of cable between the repeater TX and the duplexer by adding quality adapters then once you have it perfect replace this with a length of cable cut to the total length of the collective. I look for proper transfer of power (right amount of insertion loss [IL] ) or minimum reflected power. Note that adding an SWR or Bird meter in the TX line will likely upset the reactance (impedance) so looking for the right amount of IL is usually best (unless you get sneaky and make a special jumper cut to a magic length so that when it is added to the Bird it's either a 1/4 wave or a 1/2 wave - then inserting the combination of the Bird and that magic length jumper changes nothing). You will likely need to repeak the duplexer pass with each cable length change to insure proper power transfer and duplexer tuning. After obtaining the correct loss through the duplexer or verifying least amount of reflected power, set the notch adjustment. If you have mismatched impedances then optimizing the cable lengths will result in less through loss and likely deeper notch depths. Wacom provides more information on the procedure in this PDF document but you really want to fix the mismatches. Look up "Z-Matcher" on this web site and in Google.

In Conclusion -
Don't be afraid of tweaking the last drop from the equipment, especially the duplexer. Duplexers that are "factory tuned" are done so with precision laboratory test equipment with near perfect 50 ohm terminating impedances on all three ports. Is your antenna, transmitter and receiver a perfect 50 ohms? I doubt it. Terminal impedance drastically affects duplexer tuning. This is no big deal in tube type equipment because you can tune out reactance's with transmitter output and receiver input tuning controls, but with lumped-LC (i.e. no tune) transistor equipment you are stuck with whatever the terminal impedance is, and therefore reactance's must be tuned out by adding z-matchers, duplexer tuning, or adjusting the magic cable lengths of the interconnecting cabling.

Most quality repeater antenna systems have no tuning adjustments so here again is good reason to tweak the duplexer. After all the duplexer needs to satisfy your transmitter and receiver on a continuous basis, not the manufacturers test equipment once in its life.

Antennas without a ground plane usually don't work well for repeaters, i.e. Cushcraft Ringo and Ringo Ranger are bad choices for a duplex antenna. An AEA Isopole is an acceptable choice for a garage repeater or any nice-weather low gain repeater antenna except that it has a very flat pattern aimed at the horizon, and if at an elevated location it will overshoot your users. Hilltop sites have had excellent results with multiple dipole arrays like the DB-224, or if you have the space, a DB-228. These antennas are also DC-grounded which makes a big difference, especially when Mother Nature delivers a lightning storm.

As far as fiberglass verticals, antennas with a decoupling sleeve, like the Stationmaster or Super-Stationmaster, usually work very well. If you want long life from a fiberglass antenna, of any manufacture, you will want to side mount it and use a made-for-the-job top brace. If you live in a high wind or high icing location then you will want to look at the StormMaster or equivalent, again with the matching top brace.

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This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.