Puzzling loss of VHF signal on a rooftop antenna

[canoeboater]

I live in the west end of Alexandria city in Northern Virginia, near Alexandria Hospital and Landmark Mall.

I have a RS Pro-197 radio (GRE PSR600 equivalent) that I have been operating on a desktop with a Diamond RH77CA back-of-the-radio rubber ducky antenna for about two years. In this configuration, the radio picks up local public safety well. Most notably, it is able to pull in US Park Police comms traffic at 166 MHz. A bit static laden, but OK.

Last weekend, I finally got around to installing my Diamond D-130N discone antenna on a gable mount on the side of my house and plugged it into the radio. Oddly, the 166 MHz stuff that I expected to hear more clearly with the rooftop antenna completely disappeared. Nothing. Not a whisper.

For laughs, I plugged the rooftop antenna into an inexpensive Uniden BC80XLT handheld scanner to see how it would perform on 166 MHz. On the cheap handheld, the lost US Park Police comms traffic came booming in with nearly full quieting. Go figure.

So, there is clearly something about the rooftop antenna that is making the RS/GRE radio act strangely. I am trying to figure out what that is so that I can deal with it.

I’m wondering if the data suggests that the radio is getting hit by a strong signal from nearby and is desensitizing. There are no nearby TV stations at nearby frequencies, but there is a nearby hospital. About 3000 feet away and 125 vertical feet above us. When I drive past the hospital, cell signal gets flaky. Admittedly, that’s far away from VHF, but I don’t know whether it might be enough, or whether other hospital RF might be the problem.

So I am fishing for ideas and suggestions…….

 

[Thunderknight]

That sounds like overload/densense. I believe GRE front ends are known for that.
My scanners would get hammered hard by VHF paging. I ended up switching to CDM-1250s for VHF analog scanning.
(The commercial VHF paging hitting me was on a hospital rooftop).

Try turning on your attenuater and see if anything changes.

If you can hear several NOAA weather channels at your house, they can be used as a rough gauge too.

 

[jonwienke]

Unidens are generally better at rejecting adjacent channel interference and not overloading. I have a pager tower near me broadcasting at 152MHz. It's strong enough of a signal that it comes in on some radios across the entire VHF band. But my 436 ignores it unless I'm trying to receive within 50-75KHz of the pager frequency, even with a ST-2 antenna mounted outdoors that gives me occasional Close Call hits from 50+ miles away on 161MHz.

 

[dlwtrunked]

I live in the west end of Alexandria city in Northern Virginia, near
...

Last weekend, I finally got around to installing my Diamond D-130N discone antenna on a gable mount on the side of my house and plugged it into the radio. Oddly, the 166 MHz stuff that I expected to hear more clearly with the rooftop antenna completely disappeared. Nothing. Not a whisper.

For laughs, I plugged the rooftop antenna into an inexpensive Uniden BC80XLT handheld scanner to see how it would perform on 166 MHz. On the cheap handheld, the lost US Park Police comms traffic came booming in with nearly full quieting. Go figure.
...…….

Common receiver overload. Very possibly an FM broadcast station. Try an FM broadcast band block. A cheap one might do. (No one with a scanner should not have one.)

 

[jonwienke]

Get a cheap SDR dongle and connect your rooftop antenna to it. That will tell you what strong signals you have in your area.