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As a practical experiment, I thought I'd see if I could hear the 'Weatherman' on 151.625 MHz (business band VHF), when he was transmitting from Mt. Diablo during the recent SCORE Baja 500 offroad race. By my estimates, where I live in Pacific Beach, in San Diego, is approximately 175-180 miles from Mt. Diablo down in Baja, MX.

Equipment: My radio is a Kenwood 730H(G) business band mobile, with remote head, installed with proper gauge power & ground wires. The business band antenna on my truck is a 1/2 wave no ground plane antenna (though is attached with a metal mount to one side of my metal roof rack which is attached to the metal roof of my truck, so may gain some small measure of ground plane effect assistance from that). The antenna is a Laird single band business band antenna, that has been specifically cut to length for 151.625 MHz, and registers ~ 1.1 SWR on an SWR meter inline between the radio antenna port and the antenna. Also the roof rack on my lifted truck is appoximately 9 feet high, so the top of the antenna is about 12 feet high off the ground.

Theory: A rough guide for the range of VHF radio communications, under line of sight (LOS) conditions, excluding tropo effects, humidity etc. is:

1.42 * √ antenna height.

For 2 radio stations transmitting & receiving to each other, the range is additive.

So, the Weatherman sitting at ~ 10,000 feet on the top of Mt. Diablo, down in Baja, MX, should have a theoretical range of approximately: 1.42 * √10000 = 142 miles.

Test 1: I walked outside my apartment in Pacific Beach in San Diego, at sea level, got in my truck, selected the Weatherman channel on my radio, and could occasionally hear it break squelch, but any race radio traffic was unintelligible.

Test 2: I thought I would drive to the top of Mt. Soledad nearby, which is approximately 900 feet above sea level, which should net additional range of: 1.42 * √900 = 42.6 miles, for a total of 142 miles + 42.6 miles = 184.6 miles i.e. now approximately the real range from my location on Mt. Soledad to the Weatherman's location on Mt. Diablo. Also, being on top of a hill, the radio waves should have a better line of sight propagation path. Unfortunately, parking at Mt. Soledad was bad, and I had to park on the North side, i.e. the far side of Mt. Soledad, with the mound & the cross on Mt. Soledad blocking direct line of sight to the south to the Weatherman's location on Mt. Diablo. I could still hear the radio break squelch occasionally, perhaps more than before, but the practical result was the same as Test 1 i.e. no intelligible reception of race radio traffic.

Test 3: I was pretty sure I had heard some race traffic break through on the radio on the drive up to Mt. Soledad, and given that in general there are always some null & some coincident wave high reception spots, I thought I'd stop at a few points on the way down the hill to see if I could get reliable reception. Well, at a couple of points on the way down, I got just that, with one location about half way down the hill on the south side, where I could fairly reliably hear race radio traffic from the Weatherman on 151.625 MHz.

Some sidenotes:

- in San Diego, I think there is a cab company (maybe based in Coronado?) that I think uses 151.625 MHz

- even though I have 100W output on my Kenwood 730H(G) mobile, I didn't try transmitting, only receiving, since I didn't want to interfere with any local US-side business band traffic or race radio traffic on the Weatherman frequency down in Baja

- I tried playing with the squelch level & 'Mon' (which I believe completely unsquelches) features on my Kenwood 730H(G), but it didn't seem to make much difference (either the radio received properly & was intelligible, or it didn't). It is a bit difficult to tell though, as this was later in the day, so radio traffic on the Weatherman frequency wasn't as frequent as earlier in the day at the start of the race

- I tried my little Yaesu handeld (a VX-170 ham radio), which broke squelch about 1/3 to 1/2 of the time my Kenwood mobile did, but none of the radio traffic was ever intelligible. This wasn't unexpected. Though I have an upgraded 1/4 wave antenna on my handheld, the antenna is still smaller than a 1/2 wave, is not grounded, and was not quite as high as the antenna on the roof rack of my truck. Some time I want to experiment with a 19" or similar appropriate length rat tail loose wire connected to the antenna ground connector hanging down as a 1/4 wave counterpoise to see if that helps much

- I also tried my Yaesu 8800 dual band dual receive ham radio, with a 1/2 wave dual band ham antenna tuned for ham bands (Larsen 2/70B NMO), and it didn't even break squelch, but that also wasn't unexpected at all, as just beforehand I had tested the SWR on that antenna & it tested extremely high (SWR = ~3), and I found that I think there is a somewhat loose connection at the base of the NMO mount for that antenna, so I need to replace the antenna cable. In retrospect, I should have connected one of my mag mount 1/4 wave antennas to my Yaesu 8800, and stuck the mag mount on the roof to see how sensitive the receive section of my Yaesu 8800 mobile is, and to compare that to my Kenwood 730H(G) mobile. It wouldn't have been a fair test if the Kenwood had a proper 1/2 wave antenna tuned specifically for 151.625 MHz vs. a generic ham band 1/4 VHF mag mount antenna, but it would have been interesting to compare consumer vs. commercial electronics quality, specifically designed for business band vs. specifically designed for ham band electronics & antennas etc. Given that my handheld managed to break squelch a few times, I would hope that, with a similar antenna, my 8800 would have performed at least as well, and maybe received the odd intelligible reception here & there. If I'm not down in Baja for the Baja 1000, I'll probably try repeating the test with my 8800 with repaired low SWR cabling & both tuned business band & generic VHF and/or dual band UHF/VHF ham antennas to contrast & compare with my Kenwood 730H(G) business band mobile.

- the theoretical & practical range discussed here is mostly applicable to a Line Of Sight (LOS) propagation path, under good conditions. Obviously terrain, such as large mountains or other obstacles in the way & other factors, such as less than optimally chosen & installed equipment, antennas, cabling & wiring all affect performance & real-world practical results
 

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I am not sure where you get the 1.42 factor; it is not correct for non-refracted LOS barely clearing the horizon; the factor is 1.22. It gets a bit bigger with refraction but not by enough to reach 1.42.

Beyond that, you have to deal with Fresnel Zone clearance. This means that a large portion of the electromagnetic field has to clear the horizon, not just a simple centerline (or line of sight) path. At 2 meter wavelengths and the types of distances you are trying, adequate Fresnel zone clearance is many hundreds of feet. Without adequate Fresnel zone clearance, the received signasl will be seriously attenuated.

Additonally, at low angle of reception, there is a reflected wave from the ground that is in inverse phase from the main incident wave. For long distances, this will significantly cancel reception until you get to many, many tens of feet above ground.

So it is not a surprise at the results that you report.

Regards, Mark B.
 
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