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I live at the top of Queen Anne Hill and have been wondering what, exactly, causes the fast (several times a second) fading phenomemon I'm observing on the trunk channels associated with the Columbia Simulcast system. Signals commonly bounce up and down by as much as 20 dB over very short (< 1-second) periods of time.

First of all, why are some channels much less vulnerable to fading than others? I can always tell when I'm listening to a talkgroup assigned to 851.9875 because it flutters and fades rapidly compared to, say, 866.2875. The lower-frequency channels seem to be more prone to fading than the higher ones in general, although there seem to be some exceptions (866.3375 is pretty fluttery despite its proximity to 866.2875 and 866.4375, which aren't.)

Second, and possibly related, why don't all these channels arrive at exactly the same signal strength at all times? If they're coming from the same place, and the frequency differences aren't significant, why would one trunked-radio channel be stronger than another?

I've observed this with two different receivers and two different antennas. Neither of the antennas are great (discones at relatively low heights, with their view of Columbia Tower blocked by a nearby house) but I'm not sure why that should make a difference.

Finally, any practical suggestions for overcoming this effect? It certainly makes listening to the trunk system more fatiguing than necessary. Do the system's users notice or complain about this, or is it just me?

Frequency changes of only a few hundred khz can cause a noticeable difference in performance, especially when reflections/multipath is involved. Plus you have a minor impact (probably a few db) from the linearity of amplifiers/filters/antennas over the frequency range.

I went through a real-world test of this not long ago with Keith K7RFY where we tried about a dozen 2-meter simplex freqs between Marysville and Mountlake Terrace. Some frequencies worked great, yet a shift of a few hundred khz would distort from multipath or have fade/flutter. Later on I ran a similar test with Ben KB7ZKA in Port Orchard and had similar results.

The best way out of this (especially on UHF and above) is to use 2 antennas with a diversity receiver. This was an available option on some of the older cell phones back in the days when cell site density was pretty light.

One last thought... With simulcast, add some minor phase distortion from signals arriving at slightly different times from multiple sources. Capture-effect is never 100% especially when the signal strengths are somewhat close.

- Rich W7KI

OK, thanks, Rich -- makes sense. I'm surprised that a few PPM makes such a difference at nearly 1 GHz, but if these signals are indeed being simulcast from multiple locations, that could explain it. It's like deliberately-induced multipath!

I'll continue to fool around with antenna placement and see if I can find a spot that works better.

OFF SUBJECT, JOHN M. Do you know someone also on the hill that was into the CB craze back in the 70's went by shutterbug, we were good friends, untill I moved to CALIEEFORNIA, I've been triing to track him down, I thought you might know him!

STEVE F.
thank for any info good luck

_________________
GOD BLESS 73s

Hi, Steve --

Nope, sorry, doesn't ring a bell. I was in Oklahoma then.

Here is a map of the KC trunk system sites:

http://www.interceptradio.com/interceptnw/kctrs.jpg

At the bottom of the map there is a listing of which sites are grouped together for simulcast.

Perhaps you are getting smacked by Columbia and Capitol Hill at the same time (?)

- Rich W7KI

Sounds like that may be likely. I've been moving one of the discones all over the yard, trailing 100' of RG-6, trying to find a spot that doesn't suck. Not easy. :(

It appears that higher locations, in addition to delivering a stronger signal as expected, are less vulnerable to the rapid fading effect. That's about all I can conclude. It is workable as-is, but maybe I'll look into putting up an 800 MHz Yagi in the future.

The comment about deliberately induced multipath is well taken. The same type of multipath that can really tear-up VHF receive quality, GENERALLY helps 800 Mhz systems. Its one of the reasons (now dont get TOO worked up here MTM!) that all things being equal, 800 MHz (and to a lesser extent UHF) penetrates buildings so well. Of course, if you don't have enough sites in an 800 system, your signal strength will be less than a typical VHF system, so 800 MHz APPEARS to suffer as compared to other frequency ranges.

In any case, what is happening is a mix of signal strength variations over brief periods of time, with some frequency selective fading. Fading can affect very narrow chunks of spectrum selectively, so some channels may vary briefly. Longer terms drops in signal strength are probably due to some channels being grouped on different transmit antennas at the tower sites. Most trunk systems limit the number of channels combined into a specific antenna to between five and ten. Often, if a group of channels seem to be weaker it means an antenna position is 'bad' (at least on the bearing to your location). Sometimes it helps diagnose a defective antenna or combiner as well...

I'd drop the signal strength into your radio to see if you can manage to limit the number of sites 'serving' your radio... Reduce multipath in other words. You might also be getting clobbered by a Nextel site on a few channels. Scanners are REALLY bad at rejecting NEXTEL intermod or overload.

ss

Good to know, thanks.



Unfortunately, attenuating the signal isn't too practical because the "dips" already go below my normal squelch floor of -105 dBm. But, in any event, I just raised the discone to a point about 20' AGL and added a quick-and-dirty homebrew preamp at the antenna to overcome the loss in the 150' of RG-6 coming into the receiver, and it is sounding quite a bit better. Still not what I'd expect up here near the highest point in Seattle, but much more tolerable than before.

I haven't seen any intermod issues with the Icom R-7000 I use as a reference receiver, nor with the other rig, which is a homebrew job with a rather gutless front end. Kind of surprising considering how close I am to the big towers up here.

I live in Shoreline (Richmond Beach) and experience the same problem.

I used to live down Richmond Beach Road (NW191st) past 8th Ave NW. Even with a 40-foot antenna pole, we were still below line-of-sight view of the various radio towers for Seattle Police (my neighbor's son worked for SPD and he wanted to hear them better). If you live in that area below line of sight, I would run into the following annoying situation ~ raise antenna pole til I had a really strong signal with no multipath distortion and discover hours later or whatever time period, that signal got a lot worse. Someone in our household was studying geology and showed me how to draw a geological profile between two points, and voila' it was clear I didn't have line of sight. Then I found for my Apple II computer (this was in the early 80s) a program where you put in antenna heights in MSL and the distance to the hill obstruction distance to each site (my receiver house and the transmitting site) and it would calculate various parameters for what is called the Fresnel Effect. This is discussed at http://www.zytrax.com/tech/wireless/fresnel.htm And, there's a Fresnel calculator at http://216.239.57.104/search?q=cache:h8 ... t%22&hl=en

What I basically learned is that the program said I needed an antenna at least 180 or so feet high to overcome this phase distortion problem. If you are west of 8th Ave NW and down the hill, this is probably what you are running into.
Dennis E

I do live West of 8th Ave NW. Thank you for your detailed information. I recently had to raise my 2 meter amateur radio antenna to avoid a multipath problem.

Scott