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and 6 meter repeaters
Compiled, HTML'd and Maintained by Mike Morris WA6ILQ.
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Some thoughts on 10m and 6 meter repeaters Compiled, HTML'd and Maintained by Mike Morris WA6ILQ. |
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10 meter repeaters have a couple of uncommon problems...
First is a 100 KHz offset. Cavities are huge and rare. Several years ago I was asked
to retune one... it was three feet in diameter and eight feet tall (and the tuning rod
extending from the top was another two feet). And most duplexers need aa minimim of 4
cavities, many times more. This situation usually mandates a split site.
The second is skip. You really need to run a 10 meter repeater in tone squelch.
6 meter repeaters are similar. Cavities are smaller, but still usually 4 to 5 feet tall. Offsets can be 500 KHz, 1 Mhz, sometimes more, depends on the local coordinator and the local band plan. The repeater band is much wider, and mobile antennas rarely cover more than 500 KHz.
Local noise is a problem, and the noise floor at the repeater site can cause serious receivers desense. Rarely do you have a noise free site.
An email to the repeater-builder asked:
Just what is a Noise Blanker and why do I need one on my 6 meter repeater? I've been
told that my repeater will be worthless without it?
GE uses the term "Noise Blanker" for the circuit in the Mastr II receiver. Motorola used the term "Extender" (and trademarked the name) for their noise blanker. The overall technique has been around since the 1950s. On low band, 6 or 10 meters a properly functioning noise blanker can be much more effective than a preamp.
The noise blanker (no matter who makes it) takes advantage of the fact that noise is broadband, and two receivers a megahertz or more apart will hear the same noise. A blanker-equipped receiver has both an FM receiver on the main channel and an AM receiver parked on a nearby quiet (hopefully) channel. The AM receiver hears only noise and the detected output is converted to narrow blanking pulses that momentarily short out the FM's IF, thus canceling those noise pulses. All of this happens at the IF frequency, long before the demodulator. At least that's the plan, and usually it works. The design of some radios blank an RF stage rather than an IF stage.
Some people say that Moto's Extender doesn't work as well as GE's Noise Blanker circuit. Not having had a low-band GE repeater to play with, I can't speak to that as I don't have first hand experience. One person whose opinion I respect has over 15 years of experience working on low-band GE and Motorola commercial gear and he says that he'll take a MASTR II over a MICOR any day as a 6 meter repeater receiver just because of the noise blanker design. When he sets one up he parks the noise blanker receiver on the low end of 51 MHz and lets it run.
Motorola recommends that the extender (the AM receiver) sampling frequency needs to be two to three MHz away from the desired frequency and on an unassigned channel to guarantee that all that it picks up is wide-band noise. I've heard of commercial low-band system operators that have actually licensed and coordinated an extra channel and left it idle and unused just to park the blankers in every base and mobile receiver on it. The plan was that the blankers heard nothing but noise, and it worked until the propagation changed and out-of area signals rolled in.
When the noise blanker (no matter who makes it) is working right, it eliminates a large majority of the noise pulses that are so prevalent on low-band channels. A non-working noise blanker can literally make a low-band system unusable. The noise blanker system messes up when the AM front end hears noise that the FM receiver doesn't, or when someone starts talking on the frequency that the AM receiver is tuned to (the noise blanker input).
Bob WA1MIK added the following:
As a general rule-of-thumb, a low-band base / repeater receiver that isn't moving around is better off with its noise blanker disabled, because site noise can render reception nearly impossible. These days, there's rarely ignition / impulse noise from automobiles; the biggest noise source is digital noise from things like traffic lights and computers. A mobile radio will encounter this but for the most part, a noise blanker isn't really needed in mobile radios any more, and in some situations where the man-made noise level is so high that it keeps the noise blanker actively blanking all the time, it can be detrimental and make reception nearly impossible. As long as you can turn it on and off when necessary, that's good enough.
Most low-band noise blanker equipped mobiles split the received signal after the antenna relay/switch so that both receivers can share the antenna. Low-band receivers are available in configurations both with and without the blankers. Some radios, such as the GE MASTR II, have two receive antenna ports, but most do not. Both GE and Motorola offered high-band receivers with a noise blanker.
Note that the the noise blanker has to be a certain percentage (of frequency) away from the main channel. This means that if you put a duplexer, or even a single pass cavity tuned to the main receive channel in front of the splitter the AM receiver will not hear a thing (because the cavity pass window is so narrow), and therefore you effectively have disabled the noise blanker. I've seen a low-band repeater with a conventional 6-cavity duplexer with one side feeding the repeater receiver. There was a separate noise antenna with a separate single cavity in-line with the noise antenna and the noise blanker antenna input. Adjust the cavity for the least insertion loss that will get the job done as every dB of insertion loss in the noise antenna path lessens the effectiveness of the noise blanker by the same amount. Note that you can get away with a non-resonant antenna for the noise blanker as it's receive-only - for example a high-band antenna can be used for a 6 meter receiver. All it is listening for is local noise.
Other than increased receiver to transmitter isolation, this dual antenna requirement is the biggest argument for split site amateur machines on 6 and 10 meters. A split site configuration allows one receive antenna to be used for both the main channel and the noise blanker receivers.
There are two relevant articles located on the Antenna Systems page at this web site, one on receiver-to-transmitter isolation, the other on horizontal vs. vertical antenna separation.
I've seen one 6m repeater where they took a single-sited machine that used two antennas on the tower and split the machine. The transmitter ended up a mile away with a low power 420 MHz cross-link with beam antennas on each end. The old transmit antenna (on its own feedline), and a pass cavity were reused for the noise blanker receiver channel (all it had to hear was local noise).
John W1GPO added the following:
No matter the manufacturer, what really counts is what signals and noise are present in the IF passband where the blanking transistors are clamping (as diode shorts) to ground for the duration of the noise pulse. If only one carrier is present, and the noise pulses occur at a reasonably low rate (mostly uniformly, nearly simultaneously) across the IF passband, so each pulse is across the width of the passband, the blanking action is excellent. In today's world of switching power supplies, LED lights, etc, those noise spikes are not homogeneous, but occur nearly simultaneously at multiple discrete frequencies in the IF passband, and there also may be other RF carriers within this passband. In this case the active clamping generates intermod (by diode effect) between the noise spikes. Also, if other carriers are present, intermod products often fall on the receive frequency, making the received noise WORSE. Therefore, it is usually best to operate the receiver with the blanker off. If increased noise on the received signal is detected, try the blanker. If the noise is purely from power lines or ignition noise, the blanker should reduce the noise often by 20 dB. If the noise is worse with the blanker on then either other strong carriers or digitized noise peaks are present in the IF passband. If pulsating noise becomes present on the receive channel, and toggling the blanker makes no difference, there may be a carrier near the blanker sampling frequency, desensing the blanker, or digitized noise in the IF passband where no intermod product from blanking it falls on the receive channel, or the rate of the detected pulses exceeds the blanking limit. Because of all the limitations to blanking these days, the best solution to limit noise, especially on low-band, is to have an IF response that has minimal ripple, is symmetrical with rounded corners, and accepts nothing wider than the widest deviation desired.
BTW in most cases you DON'T need a preamp on a 6m FM receiver!!! Most already hear 0.2µV or better and I've seen a few do between 0.11µV and 0.13µV for 20dB quieting (and no, it wasn't a leaky or off-calibration generator). Without a functioning noise blanker the effective sensitivity will likely be in excess of 1µV at most sites just due to the local man-made and atmospheric noise.
John W1GPO added the following:
There is an exact way to find out if a pre-amp will help. Feed via a lossy "T" in the antenna lead a signal with the receiver first connected to a dummy load, and set the generator to a level that produces either 12 dB SINAD or 20 dB Quieting. Then replace the dummy load with the antenna, and increase the generator output to deliver the same result as before. If there is at least a 6dB difference, then a pre-amp will make NO difference, and it would probably overload the receiver. If there is a 0-5 dB difference, then adding a pre-amp will definitely help, but the benefit will be the greatest closer to 0 dB, and the least closer to 5 dB. The benefit is logarithmic with the most gain near 0, and hardly any difference between 5 and 6 or more dB. The pre-amp needs to be high quality, have a noise figure much lower than the receiver's noise figure, and the minimum gain possible to overcome the receiver's noise figure. That means something with 3-9 dB gain. Most commercial preamps have 15-20 dB gain, which is way too much and will overload any receiver. If that's all you have available, add a 10-12 dB attenuator to the output of the preamp to reduce the overall gain.
To answer some of the questions posed above, the MICOR low-band receiver strip contains an area for the noise blanker components. These are installed in mobile receivers and the appropriate cover plate allows tuning of the "Extender" receiver front-end. A switch on the rear of the MICOR control head lets the user disable the Extender. See the composite photo below, supplied by John W1GPO. Click on it for a larger view.
The MaxTrac low-band mobile radio also has its "Extender" components installed on the RF board. All MaxTrac low-band RF boards contain this circuitry. It is automatically enabled when the radio turns on. The front-panel "MON" button can be used to disable the extender when necessary. See the photo below, supplied by Bob WA1MIK. Click on it for a larger view.
The Kenwood TK-6110 low-band mobile radio also have a built-in noise blanker that can be enabled or disabled on a channel-by-channel basis. There's no manual way to turn it on or off via a front-panel or external switch. In those radios you will frequently find two adjacent channel positions with the blanker enabled in one and disabled in the other.
The Vertex VX-series low-band mobile radios also have a built-in noise blanker but the programming software can assign a front-panel button to enable or disable the blanker. GE low-band MLS and MLS-II radios have an optional noise-blanker module that plugs into the RF board. I've never seen one and it doesn't appear to be controllable, so you're stuck with it operating if you installed it. There may be a circuit modification that can add that function.
Contact Information:
The author can be contacted at: his-callsign // at // repeater-builder // dot // com.
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This page originally posted on 10-Sep-2008
Article text, artistic layout and hand-coded HTML © Copyright 2008 by Mike Morris WA6ILQ.
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.