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Question regarding Ground Plane Antenna and Airband Scanning

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Luis_C

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Well, a few days ago I started to do my first ground plane antenna, but I didn't had enough wire copper for the radials, so with a plastic disk and some coat hangers we made like a base for the antenna, it has kinda good reception, but well, I just removed that base for testing, and I just have a N-Chassis Connector soldered to a 60 CM Stiff Copper Wire, and went out because is time when there's many airplanes a few minutes ago, but it was weird, because sometimes some airplanes were excellent, but I couldn't hit the tower, simply silence, but if I pressed ATT button on my PRO-95, I could listen to it a bit, but just a little bit, couldn't be understood, as if it were overloading my scanner, sometimes distortion from the tower, but ATT would do complete silence. Weird thing is, that if I left ATT some airplanes wouldn't be heard, and sometimes without ATT on, some airplanes would be heard, but distorted, so I believe that now this really was overloading, because if I pressed ATT signal reception now was great, but still tower couldn't be heard. It was about luck. Heard some distant Air to Air, really weak, but it was heard.

I also tested police, and it would always sound great, but in some cases a bit better with ATT on. Police is on VHF Hi. (156 MHz, etc)

Why do you think it was acting like that? What's going on?

I'm getting the radials hopefully soon. Do you think it will solve this kind of problems?

BTW, I have Belden RG8X Coax, about 7 meters.

Thanks
 
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LarrySC

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Radials on a ground plane are needed for transmitting as they complete the simulated earthen ground for the wave pattern. HOWEVER, for receiving they are of little use as they only reflect signals which bounce up to the {+} element. For aircraft the discone is a good choice.
 

Luis_C

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LarrySC said:
Radials on a ground plane are needed for transmitting as they complete the simulated earthen ground for the wave pattern. HOWEVER, for receiving they are of little use as they only reflect signals which bounce up to the {+} element. For aircraft the discone is a good choice.
I see, but anyone has any ideas about why the antenna is a bit weird?

Thanks
 

ka3jjz

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Just taking a wild guess here - did you perhaps program the air freqs as FM instead of AM? The distortion you describe could be caused by that.

73s Mike
 

Al42

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LarrySC said:
Radials on a ground plane are needed for transmitting as they complete the simulated earthen ground for the wave pattern. HOWEVER, for receiving they are of little use as they only reflect signals which bounce up to the {+} element. For aircraft the discone is a good choice.
Still a perfect score, Larry.

An antenna needs a ground. Receiving or transmitting it needs a ground. The impedance of a piece of wire isn't anywhere near the impedance of coaxial cable, and the receiving pattern of a piece of wire isn't anywhere near the pattern of a ground plane antenna. (Why don't you model the "antenna" before you chew your feet instead of after?).
 

prcguy

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Antennas are reciprocal comparing transmit to receive and the ground radials are necessary in Luis’s case. Some antennas are specifically designed to operate without a ground plane but a simple piece of wire sticking out of a connector will not match to 50 ohm coax, even if it’s cut to the calculated ¼ wavelength.
The distortion problem and the attenuator improving it is the classic symptom of an overloaded receiver. Any big towers or hill tops nearby? How about a hospital in the neighborhood? Hospitals usually have high power VHF or UHF paging systems to saturate the facility (and usually the whole city) and wreak havoc with local scanners.
prcguy
 

kb2vxa

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Hi Luis and all,

Let's start with "Some antennas are specifically designed to operate without a ground plane..." just for better understanding. The term "ground plane" refers to an artificial representation of the Earth's surface, that's why the radials are horizontal. In other words they raise the "earth" up to the radiating element of the antenna. Consider the AM broadcasting tower which uses the Earth itself as the other half of the antenna and why all transmitters and receivers incorporate "antenna" and "ground" terminals one way or another. Don't be confused, the old shortwave sets used separate terminals, modern stuff usually uses coaxial connectors but antenna and ground are concentric. In the case of antennas without radials, that is those that aren't groundplanes still incorporate a counterpoise element, another sort of artificial ground for the radiator to work against.

BTW, you might do better with a less complicated mechanical design. I have always made my groundplanes from flanged chassis mount SO-239 connectors and 14AWG solid copper wire for UHF and 1/16" diameter non fluxed brazing rods for VHF, copper is soft and too easily bends and sags.

Yours sounds like a receiver problem, not an antenna problem. First of all the lower half of the aircraft band (ground control and tower) is AM and the upper half (corporate) is FM. If you have the channel programmed to the wrong modulation mode you will get the funny sounding audio that you describe. It's highly unlikely that an FM receiver will overload and distort on strong signals while AM can be troublesome if the receiver doesn't have a good AGC circuit. Tower and aircraft on the ground can sound garbled and scratchy while you're in the airport because even a low power (tower uses only 10W) signal at very close range can overload the receiver causing clipping distortion. Use this information to figure out what you're doing wrong, no doctor in his right mind will diagnose a patient over the telephone for pretty obvious reasons, we can't do any better.

PRCguy, now you're talking about intermod and that's a horse of a different color. Here we have a case of weak and/or distorted audio, not intermod. His problem isn't with "outside signals" but with those he's trying to receive. You were only confused, I hope you're not now.
 

ka3jjz

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kb2vxa said:
First of all the lower half of the aircraft band (ground control and tower) is AM and the upper half (corporate) is FM.
Not true. The entire 118-136 Mhz range is AM mode, there's no FM on that band at all.
There are several ACARS data channels in that range, however.

73s Mike
 

prcguy

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No mention of Intermod from me yet. I had to go back and read the first post, which has two distinct parts. One is antenna related and ground radials will complete that one, the other is a distortion problem, which the attenuator has an effect on. There is a piece if information missing that hopefully Luis can fill in and I was making an assumption on. Does the PRO-95 receive OK with no distortion when using the supplied rubber antenna and then get distorted with the outside antenna? If so, this would probably be caused by nearby high power VHF/UHF transmitters and is fairly common with wide band receivers and police scanners. The result causes Intermod within the receiver from compressing the input amplifier and first mixer and the attenuator is there to combat this type of problem. Intermod generated at a transmitter site is probably what Warren mentioned and that is a different topic and probably not part of Luis’s problem.
prcguy
kb2vxa said:
Hi Luis and all,
Snip....
PRCguy, now you're talking about intermod and that's a horse of a different color. Here we have a case of weak and/or distorted audio, not intermod. His problem isn't with "outside signals" but with those he's trying to receive. You were only confused, I hope you're not now.
 

Luis_C

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Hey thanks guys, yeah, I'll have to figure it myself, but a bit of help is great. All the channels are programmed AM. No problems there. Checked a few times.

prcguy said:
The distortion problem and the attenuator improving it is the classic symptom of an overloaded receiver. Any big towers or hill tops nearby? How about a hospital in the neighborhood? Hospitals usually have high power VHF or UHF paging systems to saturate the facility (and usually the whole city) and wreak havoc with local scanners.
prcguy
Yeah, that's what I thought. Well, sadly in all the place we have cell towers nearby, so that might be the source. And in some channels that I have programmed, squelch at the min, not all the channels, I hear FM transmissions, but this is in all the city, and in my house can almost be noticed, in other places is ugly.


ka3jjz said:
Not true. The entire 118-136 Mhz range is AM mode, there's no FM on that band at all.
There are several ACARS data channels in that range, however.

73s Mike
Well, actually, I found some assholes that seemed from a company transmiting at 131 Mhz FM. At least, this isn't always used, only in the night it seems.

prcguy said:
No mention of Intermod from me yet. I had to go back and read the first post, which has two distinct parts. One is antenna related and ground radials will complete that one, the other is a distortion problem, which the attenuator has an effect on. There is a piece if information missing that hopefully Luis can fill in and I was making an assumption on. Does the PRO-95 receive OK with no distortion when using the supplied rubber antenna and then get distorted with the outside antenna?
prcguy
Afirmative. No distortion with the rubber dock. And there probably is many VHF/UHF Transmitters.
 
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