Different Recievers More/Less Suceptible to Multipath?

[Birken Vogt]

Here in the foothills of the Sierra Nevada Mountains the canyons are deep and the hills are high. Fire and police agencies here use a system of high band repeaters on different hills on the same pair, selected by different input PLs. The dispatch center is also on a hill and goes through the repeater the same as any mobile unit (no remotes in other words).

The problem is we installed a repeater further up in the mountains than any previously. Up there the canyons are enormous, the slopes are steep and there are lots of reflective granite rock faces everywhere.

The repeater antenna and the command center antenna are pure, literal line of sight to each other. However our initial indication when we installed it was that multipath distortion was pretty bad from the command center to the repeater, and they indicated they were having a hard time understanding too.

It was raining really hard that day and there was no place dry to work so we could not do any real testing. I have not heard back from the customer since then on how their field tests worked.

Based on my limited understanding of multipath distortion, it is due to some sort of AM component overwhelming the IF amp and getting into the discriminator. It set me to wondering, would certain recievers be more or less suceptible to multipath than others purely on their design and operating characteristics? I guess it would have to be trial and error. Current radio is a TKR-720 by the way.

Failing that, another idea that I had wondered about was turning the transmit power of the command center way down. A 1 watt HT with a rubber duck will activate all repeaters just fine from up there and I suspect they are running 50 or more watts though I cannot confirm this yet. And on the same note padding down their recieve line, though it sounds strange, they are line of sight to all the repeaters, which all put out over a hundred watts into gain antennas mounted high on towers.

For what it's worth the signal path ranges from 4 miles to 17 miles (the one in question).

Birken

[k2hz]

Based on my limited understanding of multipath distortion, it is due to some sort of AM component overwhelming the IF amp and getting into the discriminator. It set me to wondering, would certain recievers be more or less suceptible to multipath than others purely on their design and operating characteristics? I guess it would have to be trial and error. Current radio is a TKR-720 by the way.

Birken

The main effect of multipath is phase distortion. The reflected signal is out of phase with the direct signal. There could be some slight differences in receiver discriminator characteristics that would affect the received signal quality.

The best theoretical solution to multipath is a diversity receiving system where the receiving antennas are spaced so that a poor multipath signal on one would likely be ok on the other. This was part of the design of early cellular systems but it is very rarely used in regular land mobile.

A receiver voting system with a remote receiver at another location is the more common solution for land mobile multipath "dead spots".

These are solutions for problems reception of mobile units into the repeater receiver. You indicated it is the control station that is the problem. In the case of a point to point system, it is necessary to have a good, reflection free path. This could mean just a slight relocation or reaiming of the control station antenna and/or use an antenna with better directional characteristics. If this does not work, it may be necessary to do a path profile between the sites and determine if the control station antenna height need to increased to clear an obstruction.

Power reductions will make things worse, not better. You need to clean up the path problem to reduce the reflected component. If the path is truly line of sight, I would suspect the antenna is aimed wrong or is not sufficiently directional.

[Birken Vogt]

Well, I did a path profile and it is sort of inconclusive. Sometimes it indicates that the signal path is just kissing an obstruction. Other times it does not. The resolution is not sufficiently high to cover every single foot of the way. Although if it indicates an obstruction at any point, I am sure there is one to some degree. When I said pure line of sight it is true to the top of the control tower, however the antenna is mounted further down. I guess I will have to visit the control tower and visually verify that my parameters are correct. If they are then it should be a simple matter of raising the antenna I hope, if there is space available, and if politics will allow, etc. This antenna I believe is a folded dipole, probably 2 bay but could be 4. All the other repeaters are arranged around it. If I aimed it at the repeater in question the others would still get plenty of signal but the gain of a pair of folded dipoles in phase is not that much. At the repeater itself we have a 2-bay antenna. The signal from the control is coming in about 45 degrees off the front of the pattern but the gain is still high there and we have to aim it that way to cover the mobile units, since they are in a difficult spot to cover, and that is what it is there for. Plus we like to keep the signal out of the neighboring county which is just behind the ridge.

You indicate that less power and padding the rx will make it worse, if that is true, should more power and improving the rx make it better?

Of course, it could also be that the control point's antennas are arranged with a recieve antenna up top and then the transmit antenna down about 40 feet with all the high band transmiters combined into the lower one and the recievers multicoupled from the upper one which is usually how it is done around here, in which case things will be a bit more challenging.

Birken

[k2hz]

More or less power is not the problem, you need to fix the path.

After my previous theoretical treatise on multipath I got thinking about my similar experiences in my 40+ years working on radio systems.

Murphy's Law for fixed control stations: The location where you want to mount the antenna is probably a dead spot!

I recommend going up on the roof or climbing the tower where you want to mount the control station antenna with a portable and make some tests for signal strength and quality. This can help indicate a possible problem before you go the expense of running the cable and mounting the antenna.

The control station antenna should be a yagi or corner reflector with good directivity. When the antenna is installed it should be aimed by monitoring signal strength and quality at the receiver.

If there is a problem, sometimes raising or lowering only a few feet or even inches makes the difference. Some times you will find that aiming at an off path azimuth gives the best results.

Even on a line of sight path, there can be significant reflections. In microwave path design, you consider Fresnel Zones which are the clearance between the main path and possible reflection points that would produce phase cancellation at the receiver. If sufficient line of sight clearance exists, sometimes lowering the antenna below the critcal point is the solution.

FCC rules require a directional control station antenna with an exception if repeaters at multiple sites are controlled from that station. If there is any multipath problem, a highly directional antenna at the control station is essential. You may need to use a dedicated control station and antenna for that particulr repeater site. A folded dipole control station antenna is asking for trouble unless you have ideal paths.

[k2hz]

Of course, it could also be that the control point's antennas are arranged with a recieve antenna up top and then the transmit antenna down about 40 feet with all the high band transmiters combined into the lower one and the recievers multicoupled from the upper one which is usually how it is done around here, in which case things will be a bit more challenging.

Birken

It would be very unusual for a control station to be set up for duplex and to have separate RX and TX antennas. It would be unlikely that the control stations would be on TX combiners and RX multicouplers with the site base or repeater stations. Remember that the control station frequency pairs would be the reverse of the normal base or repeater pairs so they would usually need to be on a separate antenna system with good isolation from the main antennas.

You are correct that you do not want to change anything at the repeater site, the "fix" has to be at the control station.

What I would do would be to first verify what the control station antenna configuration is and that the control station radio equipment, audio line and console port for that system is OK and it really is an RF multipath problem.

Next would be to verify the control station antenna system components are OK. I have seen problems where the apparent multipath is due to the fact that the coax is bad, open connector, bad antenna, etc so that the signal is not making it to the antenna and the path is only by leakage from the coax.

Then, if you can connect the control station temporarily to another antenna, try that and see if it is better. I would also go up to the roof with a portable and see how that works compared to the control station and maybe look for a location where you get a very good signal to and from the portable. If you really have a good line of sight from the building, a lower antenna location may be the solution.

Good luck!

[Birken Vogt]

Well let me give a little further information. The control point is a CDF emergency command center set on its own little hill with its own tower right next to the building. It is a very modern building and just had a complete radio upgrade. However I have never had a reason to inspect their vault though I go there now and then to talk to people, etc. Anyway, it is no different really than any repeater site. On high band they transmit and recieve on 12 repeater pairs that I can think of right off the top of my head and I think there are a couple more that they use too. So even though they are not full duplex on any one pair of channels, they may as well be, because they have to be able to listen to all of the others when they transmit on any of them. This is why I am pretty sure they have a dedicated recieve antenna up high, and they may even have more than one transmit antenna since they are licensed for 100 watts on every channel though I don't know if they actually do that much power.

Suffice it to say that making changes here is not so simple as throwing up a new antenna Interaction with every other system must be considered and a tower climber must be hired. I will also have to deal with the state bureaucracy and get the state radio techs involved, who can be months behind in their work....

Not trying to be pessimistic but realistic. It looks like the best option if the situation needs correction might be to install a wireline tone remote at the site. The site is very remote but a buried telephone cable runs right by it if you can believe that and I think there might even be a place for the phone company to access it nearby. It is really an ideal site, top of the tallest hill around, covers the area in question the best of any site, power line runs right by it, a building with power was already there, phone line intersects the power line there (what are the chances of the phone line intersecting the power line at 90° right on top of the hill????!!) a good road to it so long as it's not buried in 12' of snow, a nice 120' tree was what we used for a tower, and nature had already topped the tree for us. Amazing. But as you said Murphy had to get his due.

I would really prefer to use a 460 MC link if it proves necessary but I think cost might prohibit that. Not only would it require all the radio equipment at both ends but also voters because as I said this system also runs several other repeaters on the same pair, selected by tones.

Birken

[Dan562]

What's the Antenna Gain and manufacture mounted on this 120 foot tree located on the snow covered mountain? Did you ever think about having the repeater antenna providing "Murphy's Law of Umbrella Effect RF Radiation?" Some times a combination of too much repeater height and antenna gain can create more signal coverage problems.

[Birken Vogt]

Sorry about that, it is the 2-bay folded dipole made by Celwave (equivalent to a DB222) with the elements phased together so it points at the town (6dB). The signal from the control point comes in about 45° off the front of the radiation pattern but the gain is still at least 4 or 5 dB at that point. I think the repeater works really well in its intended coverage area. Just that the link between it and the control point suffers from the multipath.

I haven't got enough information to decide how to attack the problem, or even decided if there really is a problem yet. Just trying to gain a little further understanding of the phenomenon, since I know it has been a problem previously with in that area.

Thanks for all the help so far. Right now we are getting the most powerful storm I can remember in years, I hope the antenna stays up...or even the tree hi hi

Birken

[psapengineer]

Just let me ramble for a few paragraphs:

The system is a VHF land mobile system. The path from the repeater(s) to the mobile changes all of the time since the mobiles are, by definition, in motion. The odd portion of the system is the control path(s) from your control station to the repeater(s). It is really a "point to point" path within the land mobile system. Therefore, all of the issues surrounding "point to point" path design are in play.

As others have mentioned Murphy is in full force with this kind of path. One can be unlucky enough to place a pair of fixed antennas exactly in two places that won't work.


So, here are some thoughts:

Does the path have line of site plus first fresnel clearance as it grazes the obstruction that is in the path? A diffraction pattern caused by the grazing of an obstruction could be what you're hearing; it sounds like multipath. Without seeing the path profile we couldn't tell if this is a potential problem but at VHF the path needs to clear the obstruction by a distance measured in hundreds of feet; not just a few feet.

Since the path is short there should be enough signal level left that you can play with swapping antennas on the tower at the office. If you have another VHF antenna try swapping the two for a test. Does that change or eliminate the problem? If so, it's likely multipath. You might be able to adjust the office antenna position a few inches (8-16" or so) and hide the problem. Of course, you have the potential to create a problem with one of your other control paths so test all of them after you make any changes.

Is the office antenna side mounted? If so, consider a possibility that the side mounting has set up a null in the direction of the given repeater or is itself the cause of the multipath distortion.

Is this in a dispatch console environment? Could two receivers, at the office, voted against each other, solve your problem?

Ok, enough rambling and good luck. Bob

[Birken Vogt]

Well it definitely does not have first fresnel clearance for much of the path. And it grazes at least one obstruction; Radio Mobile tells me that it does so by .26 meter at one point at least. I have made a jpg of it here I hope this works:



Hope everyone can see that

Birken

[psapengineer]

Uckkkk, the path does look badly diffracted.

Diffracted paths over one knife-edge obstruction can be stable. That doesn't appear to be the case in this instance since there are multiple obstructions and none of them are knife-edge.

Unrealistic solutions exist (like 1000ft towers) but I would hazard to guess from afar that you need a system level solution to fix this. I would consider something like a control path to a hill that has a different aspect to the obstructed site.

Good luck. Bob