Transmitter sites on the side of a hill

[joebtsflk1]

Thought I would make a new topic for this...posted this in earlier topic of mine Roadless Transmitter sites.

Just received word today that preferred mountain top is off-limits. So, have to look at a site down the hill and onto a different parcel

It puts the site 600' closer to juice. It still has clear LOS to city of license, plus the hill would provide some terrain shielding in the direction of the primary population node. So, while the theoretical coverage in that direction would be little affected, there would be less signal leaking through the saddleback down into the valley. Better for an on-channel booster.

Big question though is would this make too much multipath? FWIW, the area has some trees, so it isn't total baldy top.

Thanks in advance...

 

[Bill Betten]

Run the site with this and see for yourself. http://lrcov.crc.ca/main/ Pm me if you cant figure out the receive portion.
Guy

 

[TomT]

Being in hill country, it's better to be on the side of the hill looking at the town, then being on a hill above the town--and shadowed towards town.

That being said, you can minimize the bad effects by using a solid state transmitter with low AM noise; and don't get too aggressive with the processing. Multipath tends to exaggerate defects in the FM signal.

 

[DanStrassberg]

Clearly, the intention of this thread was to discuss FM sites. However, I have never seen an AM site like that of WDEV 550 Waterbury VT. It's a three tower AM DA that is built on the side of a hill. The bases of the self-supporting towers are at very different elevations. If anyone who reads this is familiar with WDEV and its most unconventional AM site, I'd like to hear about the problems in designing the array and proofing it. The vertical radiation pattern is probably very interesting.

 

[fm-engineer]

Tom,
I would be interested to know some specifics on your experiences involving solid state FM transmitters and low AM noise vs. tube transmitters in multipath prone environments. About eight years ago I did a graduate study on incidental noise in FM transmission paths, and also the effects of multi-path vectors in FM receivers. In the early days of FM stereo transmission, the incidental noise goal in the field was -40dB in a properly operating transmission path. Today, one should expect a properly tuned transmitter, system path, and antenna to produce noise as low as -50 to 55 dB. In some cases best efficiency does not equal best noise, and vice versa. We used to load tubes pretty heavy to produce the best noise measurements. Resonance tuning that produces the most power will usually be several dB higher in noise, at least in most tetrodes and pentodes. Harris factory test data on modern tube type transmitters is usually around -50 dB into calibrated loads. Solid state transmitters are capable of even lower noise measurements, especially the on the asynchronous side.

Most receivers are able to reject a significant level of incidental noise, but our study indicated no real reception improvements with noise levels dropping from -45 through -60 dB. The receivers began to produce distortion when the transmission noise reached -39 dB, again with a good amount of multipath vector applied. -40 is 1%, so anything above seems to be the key factor. The terrain multipath vector model applied was for hilly terrain ranging from 800' to 3000'.

Being in hill country, it's better to be on the side of the hill looking at the town, then being on a hill above the town--and shadowed towards town.

That being said, you can minimize the bad effects by using a solid state transmitter with low AM noise; and don't get too aggressive with the processing. Multipath tends to exaggerate defects in the FM signal.

 

[TomT]

Observational only. We had an FM 2.5 H (modified to eliminate the IPA--well, one holiday weekend the IPA eliminated itself--final fed directly by an FX 50 exciter). 3 bay ERI antenna at a site in the Ohio River valley. Processing an Omnia 3T with classic rock. This was the 3rd FM into a town of 5000, hence the initial bare bones install with an old transmitter; the original set-up only required a TPO of 1100 watts. We later increased from a 3KW Class A to a 6 KW Class A when a 1st adjacent station moved farther away from us.

This set-up replaced by a Nautel 2500 watt transmitter. Same processing, same exciter. The 2.5 H required 25 watts drive to make full power, the Nautel's two 1500 watt amplifiers required total drive of 40 watts.

Result was a noticeable decrease in observed multipath in those locations where it was particularly annoying before.

What we have in this area is the narrow river valley where everyone lives, at 600' AMSL. The surrounding hills--geologically a "dissected plateau," run between 1200 and 1400' AMSL. Our site is at 1260', with the antenna up just under 200' ag. Our transmitter site is on the edge of the hills overlooking the valley, so we can see most of the COL from the site. However, there are two main roads in the area, the one on the Ohio side, where our transmitter is located, is under the edge of the hills--this is where we notice the most multipath, as even close in there is no line of site path to the antenna anywhere along this road. The multipath varies summer to winter, worse in winter with the leaves off the trees.

There's no doubt that some tube transmitters can be tuned for good noise figures. Keeping them there can be fun. Our 2.5H just barely met required TPO of 2450 watts--happier running at 2300. I have no doubt that the better new solid state transmitters will easily beat these noise figures, and won't require so much maintenance. In this case, I was comparing a 1974 transmitter, which had been changed in frequency 3 times (99.3 to 89.5 then to 92.3--it's "still going" at 90.3 in Indiana) to a brand new Nautel. Given the prices for low power solid state transmitters nowadays I would always choose solid state over tube.

 

[BobOnTheJob]

Incidental AM & a solid state vs tube IPA. Continental 816R2. ERI tends to tune their antennas a little high in frequency so that when ice starts collecting, the reflected power drops before it starts to rise. That necessarily distorts the bandwidth as the VSWR will be higher on the upper sideband & lower on the lower sideband and in the process it will create some Incidental AM. Here's the catch...with the original tube type IPA in the 816R2, the tuning prior to the PA stage was narrow enough that it could be tuned to cancel out the antenna's inequality. This goes back 25 years, but I recall getting numbers in the mid to maybe upper 40's with the tube IPA. When Dave Chenowith came to town & retrofitted the new Solid State IPA, the tuning was so broad that it was no longer possible to accomplish that & the result was actually poorer numbers (-40 or so?). Keeping the mid-upper 40's required keeping the shack temperature constant, so it required re-tweaking as the Indiana seasons changed. Another downside of the Solid State IPA was that if the PA input tuning was far enough off, it would become unstable...a problem never noted with the tube type IPA. Solid state has come a long way, but I question whether a broadband "no tune" Solid State rig will be able to achieve great Incidental AM numbers on an antenna that's tuned a couple or three hundred kilohertz higher than the licensed frequency.

 

[BobOnTheJob]

it's "still going" at 90.3 in Indiana

Indeed it is. Columbus Indiana's "Christmas Station" is bring yuletide joy to a city of about 35000 people via that venerable 2.5H!

 

[fm-engineer]

Thanks for sharing your experience. I studied this at length for several years, and I would choose a solid state for new installations just for the reliability factor. You can't beat Nautel For existing tube rigs -45 to -50 incidental is a good number. -50 or -55 asynchronous is also excellent, and can be accomplished using a hum-null circuit. I prefer three-phase closed delta circuits, which in this area produce excellent low-noise power for single and three phase demands. The ERI antenna (if a roto-tiller) should be fairly broad, and in most cases will appear pretty linear to the transmitters. There are some old ERI (Gates) ring-stubs out there that are very narrow. Gates sold a ton of them to Class A's back in the early 1970's, combined with the 2.5 transmitters. That ring-stub antenna was very narrow and problematic. Too broad can introduce problems too, but that is a topic for another day.

There's no doubt that some tube transmitters can be tuned for good noise figures. Keeping them there can be fun. Our 2.5H just barely met required TPO of 2450 watts--happier running at 2300. I have no doubt that the better new solid state transmitters will easily beat these noise figures, and won't require so much maintenance. In this case, I was comparing a 1974 transmitter, which had been changed in frequency 3 times (99.3 to 89.5 then to 92.3--it's "still going" at 90.3 in Indiana) to a brand new Nautel. Given the prices for low power solid state transmitters nowadays I would always choose solid state over tube.

 

[fm-engineer]

Yes, they can, but only if the ERI is tuned properly. The trick is to keep the return loss low at carrier and above. Moderate icing will cause a tune down and you are still ok. You can get in trouble with antenna that is mis-tuned to produce a narrow cavity on carrier.

-60 dB or better is common on a few of the systems I work with that have Nautel and Armstrong trasmitters. That is using both Shively and ERi antennas.

Solid state has come a long way, but I question whether a broadband "no tune" Solid State rig will be able to achieve great Incidental AM numbers on an antenna that's tuned a couple or three hundred kilohertz higher than the licensed frequency.