[Fwd: [RFI] 1500 Watts vs. Radio Shack Amplified Antenna]
Sat, 08 May 1999 18:34:16 -0700
Last submission apparently didn't get posted...let's try this again.
> In all probability what is happening here is that at high power levels there is
> enough RF coupling into the active antenna to saturate the active element in the
> pre-amp. The phenomenon is called "cross-modulation", and basically turns the
> pre-amp on and off in time with your modualtion, thereby modulating the intended TV
> signal with your transmissions (ever drive by an AM broadcast transmitter & hear two
> stations at once...the same phenomenon). There is typically a threshold effect,
> which is why you don't bother it running barefoot. It's probably not harmonic
> coupling, so the presence or absence of a low pass filter on your transmitter won't
> affect the problem.
> I'm presuming that the power supply for the active antenna is one of these things
> that sits near the TV and superimposes the DC power on the coax going up to the
> active antenna. In this case, it is also possible that your RF is affecting the
> power supply rather than the antenna pre-amp. If the power supply output varies in
> time with your transmissions, it would apply amplitude modulation to the pre-amp and
> have an effect very similar to saturating the pre-amp's active element.
> The only way to fix the active antenna is to keep your RF out of the pre-amp's (or
> power supply, as the case may be) active element. Adding some ferrites to the
> feedline at both the antenna and power supply ends is certainly worth a try. Just
> remember that it takes a lot of inductance to have any significant effect at HF.
> You'll have to either use a number of cores slid over the coax, or else wind several
> turns of the coax through a larger core. It doesn't have to be tightly wound...a 6"
> diameter or so loop with half a dozen turns thru a large ferrite core would be a
> good place to start. Also make sure the installation uses good quality coax with
> good quality connectors, and that all the connections are tight.
> If your RF is getting into the pre-amp through the antenna elements, there isn't a
> whole lot you can do about it. The right fix in this case would be a high-pass
> filter between the antenna element and the pre-amp, but that probably isn't a
> reasonable retrofit. You are correct that a high-pass filter at the TV is not
> likely to make any difference. A different antenna (hopefully one that doesn't
> require a pre-amp) might help, though.
> Keep in mind that as long as your station meets the FCC spectral purity requirements
> and you're not exceeding the legal power limit, you have no obligation whatsoever to
> fix interference to your neighbor's consumer electronics. You can download the FCC
> interference handbook from the FCC website (I think it's also on the ARRL website)
> and show your neighbor what the rules are, and that the fault lies in their TV
> system, NOT with your transmitter. You might also refer them to Radio Shack,
> although I suspect they won't get much more than a big runaround. Making sure they
> understand that anything you do to help is just because your're a nice guy and
> interested in being a good neighbor should go a long ways to alleviate any future
> Good luck,
> Keith KB6B
> WBH3@daimlerchrysler.com wrote:
> > My neighbor who lives 55 feet away from me has a Radio Shack Amplified Antenna.
> > This thing looks like a real big Frisbee.....it is round and has a wide band
> > amplifier built into the housing. No problem with my Icom 765 running 120
> > watts, but when I turn on the L-4 B, I obviously totally wipe him out.
> > Personally, I don't think there is any possible way I can prevent this from
> > happening. I don't know how wide the pre-amp is but it most probably picks up
> > 20 meters. So.....with a TV signal in the microvolts being mixed with a 10 thru
> > 20 meter signal in the 1000+ watts range being mixed together in the pre amp,
> > there isn't any doubt in my mind what gets presented to the TV set. I really do
> > not see any hope for this situation. I cannot get to the preamp and I don't
> > really think putting a high pass filter at the TV set will do any good either.
> > I of course am using a low pass filter. I haven't tried lowering my power from
> > the L-4B lower than about 700 watts and I still cause problems. Other than
> > offering to buy him a cable TV installation, does anyone have any suggestions
> > how to handle this situation?
> > Thanks,
> > Bill WX8S
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