Technical Question

[donnyjaguar]

I posted the below on a couple of other groups. I thought it might be more suitable for this groups' experiences:

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Greetings all. I have a curious problem. I *think* I've figured it
out but wanted to bounce my theory off of the more learned.

I installed my TV antennas on the weekend. Before I did so, I pointed
my rotator to 121° (true north). This being the bearing of the side
of my house as per Google Earth. I installed everything and made
sure the VHF and UHF antennas were aligned with the side of the
house. Not hard to do when you're up a tower. I made sure not to lay
the rotator on its side as I know they'll sping if they aren't
vertical. I installed the UHF preamplifier and all the cables etc.
I guess I should mention that the preamp is a Winegard AP4700 19dB

and the UHF antenna is a Delhi CYD1470 (wideband, ch 14-70 approx

13dB). The VHF antenna is a hi-band 20$ special with no preamp. Each

antenna has a separate feedline going down and are joined with a

diplexor/splitter. The power insertor for the UHF ant/pre is in the

right place. LMR400-75 cable (3/8" semi rigid) is used for low loss,
similar to RG11.

To simplify the explanation, I'll describe the bearings of the 2
groups of TV channels that are my primaries:

CN Tower(Toronto) bearing 135°, distance 23 miles, 12UHF channels
(analog & digital)
Buffalo NY bearing 145°, distance 100 miles, 8UHF channels (analog
& digital)

Follow me so far? Here's the rub.

To bag the best DTV signal for WGRZ in Buffalo, which is the weakest,
I found I have to point the antenna at about 184°, as per the rotator
control box. I just assumed that I screwed up the initial bearing and

was going to twist the mast 40° to bring it back in line. Nope! Its

really pointing at 184° as per Google Earth. I used my GPS to onfirm.

On VHF the bearings appear to be bang-on so I ask myself, what's
going on here?

I have a theory. The UHF preamplifier is getting overloaded with the
dozen "local" stations and its not until I steer the antenna away
from them that there's enough signal drop for the amplifier to
improve the signal of my "DX" stations. As an interesting bonus,
sort of, I have one station in Hamilton coming in perfectly because
its actually at 184°.

I welcome your thoughts on this - and a strategy for resolving. Mind
you, all are coming it well enough to watch anyway but I have to
admit I don't feel like I'm getting my money's worth here.

DJ

[Russ-PA]

It sounds as if you're onto what's probably happening. I am presuming that it is only this one channel you have this problem on. It is entirely possible that strong signals closer to you and also on either adjacent channel could be causing you to have to null them rather than peaking exactly onto the desired signal.

But there is also another possibility, which not being familiar with the stations in question, I can't quickly answer, but others here can, namely that the location of the transmitter may be outside of Buffalo enough to significantly alter your direct peak bearing, given what is not a really great distance.

[jwiles]

DJ.

Might want to recheck the variation/declination in your area. It is probably about 20 degrees or so and to get true north, you probably want to point your rotor/antenna at about 20 degrees magnetic rather than 121 degrees.

John

[Mark]

I would agree that the preamp is the problem, with that good cable and antenna I'd argue you really don't need one.

You should be able to see the problem viewing/scanning through the analog UHF channels where the overload would be visible as weak images of stations on empty channels where they should not be, and possibly seeing interference to weaker analog channels from the stronger channels.

[donnyjaguar]

Just an update. I moved the preamplifier from the top of the tower to the bottom. I then put a 7dB attenuator inline between the antenna and the preamplifier. This seems to take care of the problem. It seems a little goofy attenuating an antenna going into a preamp, but it does work better than removing the preamp altogether.