Aircraft monitoring with base antenna on Uniden scanners

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toad99

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I have an attic mounted discone antenna that I have used to feed multiple Uniden scanners. I noticed that my 996T and 780XLT could pick up aircraft frequencies that my 996XT, 536HP, and SDS200 could not. I eventually discovered the problem was my local channel 13 television station. I made a 212 mHz open coax stub (8" of RG59) attached to a tee where the antenna connects to an active coupler. This reduces the TV signal by about 20 dB. Now any of my scanners will work perfectly on air band. My test signal is a 2 watt ASOS transmitter about 5 miles away, which is not audible when I disconnect the stub. I suspect the TV signal would also affect a handheld in a similar manner. My 396T is much better than my SDS100 on air band at my location.
 

toad99

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Correction: The SDS200 does not seem to be afflicted. My SDS100 and 436 connected to the base antenna are desensitized (as would be expected) as well as the 536 on air band without the stub filter.
 

toad99

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Yes, my 780XLT was able to pick up a listenable airband signal that my Icom 8600 could not quite monitor, connected to the same antenna.
Pretty impressive!
 

n1chu

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On a related matter, I have been looking for a yagi air band antenna without success. So far the only thing I can find is a commercial grade yagi antenna that is in excess of $1000, and a link to a ham’s website that shows how to construct a home brew. I was wondering if anyone has modified a 2 meter ham yagi antenna to resonate at 120-125 MHz? Currently running a scanner beam (Bob Grove of Monitoring Times Magazine fame took a Radio Shack directional TV antenna, modified it and sold it as a scanner beam. When Bob retired, Scanner Master picked up the remaining scanner beam stock. I don’t know if they are manufacturing more or not. I am on my second one... they are not very sturdy and I’d like to swap it out for a gain directional air band antenna that won’t break the bank and I don’t have to build.
 

n1chu

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On a related matter, I have been looking for a yagi air band antenna without success. So far the only thing I can find is a commercial grade yagi antenna that is in excess of $1000, and a link to a ham’s website that shows how to construct a home brew. I was wondering if anyone has modified a 2 meter ham yagi antenna to resonate at 120-125 MHz? Currently running a scanner beam (Bob Grove of Monitoring Times Magazine fame took a Radio Shack directional TV antenna, modified it and sold it as a scanner beam. When Bob retired, Scanner Master picked up the remaining scanner beam stock. I don’t know if they are manufacturing more or not. I am on my second one... they are not very sturdy and I’d like to swap it out for a gain directional air band antenna that won’t break the bank and I don’t have to build.
 

Ubbe

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The 780 uses tunable tracking filters in the airband, a narrow bandpass filter that follows the frequency you are monitoring in the 118-175MHz band. No other scanner that I know of has that and scanners generally use one wide filter that are fixed and bandpass the whole 118-175Mhz band.

The 536 has two filters, 118-137 and 137-220 so it is a bit surprising that it cannot attenuate a 212MHz signal 20dB from its 118-137MHz filter.

/Ubbe
 

Ubbe

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I was wondering if anyone has modified a 2 meter ham yagi antenna to resonate at 120-125 MHz?
It's always difficult to lower the frequency on a yagi antenna. You'll need to make the boom longer, or remove elements, and the elements has to be longer and the space between them must be wider. It's probably easier to take a FM broadcast antenna for the 88-108Mhz band and cut the lenghts of elements and drill new holes for the elements. It should also sell for far less money than an amateur antenna.

/Ubbe
 

n1chu

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It's always difficult to lower the frequency on a yagi antenna. You'll need to make the boom longer, or remove elements, and the elements has to be longer and the space between them must be wider. It's probably easier to take a FM broadcast antenna for the 88-108Mhz band and cut the lenghts of elements and drill new holes for the elements. It should also sell for far less money than an amateur antenna.

/Ubbe
Thanks!
 

n1chu

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“It's probably easier to take a FM broadcast antenna for the 88-108Mhz band...” Thanks again. I’m hoping someone has done what you suggested and will share the parameters...
 

rcool101

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My 2¢.... Doesn't make sense to me to use a directional antenna on a moving target unless pointed at an airport
 

n1chu

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I’m more interested in learning the results of someone who mast have done it. But I’ve only researched the availability of a ready made antenna. I will now go looking for the possibility someone took an existing antenna and modified it... however, I don’t have much hope I will find someone who did this. So, the next thing is to search for the calculators you mention.
My 2¢.... Doesn't make sense to me to use a directional antenna on a moving target unless pointed at an airport
Nothing like stating the obvious. But you’re right. The fact that I need a little bit more gain to capture the airport was “assumed” or “implied” was not spelled out in my quest for a sturdy, increased gain directional antenna... (or maybe I do have Patriot Missile radar tracking abilities linked to my rotator!)

Truth be told, it’s the airport I can’t hear. It’s just a tad below the “normal” noise floor for my location. I don’t have any offending noise generation issues and adding an antenna amp increases may increase the signal strength but at the same time it also increases the noise floor, giving me a net gain of zero for the effort. The other services I listen to (VHF High, UHF and 800 MHz) are strong enough to be heard using an antenna cut for the air band so I figured I’d just point a yagi at the airport and lock it down. FTC
 

dlwtrunked

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Truth be told, it’s the airport I can’t hear. It’s just a tad below the “normal” noise floor for my location. I don’t have any offending noise generation issues and adding an antenna amp increases may increase the signal strength but at the same time it also increases the noise floor, giving me a net gain of zero for the effort. The other services I listen to (VHF High, UHF and 800 MHz) are strong enough to be heard using an antenna cut for the air band so I figured I’d just point a yagi at the airport and lock it down....

Your understanding or noise, pre-amps, etc. is a little weak. Noise is not just something there that a pre-amp will also amplify. There is also reciever noise. The goal is to use a pre-amp with low noise to amplify the signal while improving the signall-to-noise of the signal. Yes, it may amplify some noise, but some (most possibly) you see at the moment is not that noise,.. If you want to test that, diconnect the antenna, does the noise change? Probably not--a pre-amp is not going to amplify that noise. Yes, adding a pre-amp can be tricky an result in receiver overload and requires experimentation. Also, are you using a good FM broadcast band bloc> No? If you do not think you need one, you are probably wrong. Strong FM broadcast signals can de-sense a receiver without giving a hint they are doing it (they decrease the scanner's gain even though you do not even hear the FM station in the audio). If you want to experiment with a yagi, using an FM broadcast band yagi (cheap) is the way to go, not a 2-m one as it is easier to shorten elements rather than lengthen them if you go that far.
 
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n1chu

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Your understanding or noise, pre-amps, etc. is a little weak. Noise is not just something there that a pre-amp will also amplify. There is also reciever noise. The goal is to use a pre-amp with low noise to amplify the signal while improving the signall-to-noise of the signal. Yes, it may amplify some noise, but some (most possibly) you see at the moment is not that noise,.. If you want to test that, diconnect the antenna, does the noise change? Probably not--a pre-amp is not going to amplify that noise. Yes, adding a pre-amp can be tricky an result in receiver overload and requires experimentation. Also, are you using a good FM broadcast band bloc> No? If you do not think you need one, you are probably wrong. Strong FM broadcast signals can de-sense a receiver without giving a hint they are doing it (they decrease the scanner's gain even though you do not even hear the FM station in the audio). If you want to experiment with a yagi, using an FM broadcast band yagi (cheap) is the way to go, not a 2-m one as it is easier to shorten elements rather than lengthen them if you go that far.
All good points, (even though all of this is off topic). But my understanding of the amplification effects is sound. Ive played with two in the past and agree with those who speak of caution when using an amp. And without getting into a narrower discussion, my presentation here is broad concerning amps. I don’t profess to be the last word on the subject but for my application I found that amps aren’t the solution.

I would like to add that my application might incorporate an antenna switch in the future, allowing my switching between a discone and the air band yagi, with the hopes the switch does not induce an amount of attenuation that is unacceptable. But that’s after I am able to achieve respectable signal improvement from the airport tower. (I am finding this broader discussion as interesting and wish to thank all who have ignored my request to stay on point! One is never too old or educated to learn! So, thanks for the constructive assistance in advance.
 

Ubbe

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If you have a perfectly tuned antenna and a zero loss coax feed and a professional grade receiver you won't see any improvement using an amplifier. But if the antenna have swr or you use it outside of it's tuned range then an amplifier will help. The worse the antenna the more it will help, according to my tests. If the coax have attenuation then it will also help to compensate for that. If the receiver has a worse noise figure than the amplifier then it will help there as well.

It's perfect for scanner use when monitoring different frequency bands and the antenna isn't tuned for that and certanly useful if the coax attenuates the signal. Even if that is perfect the scanner probably have a bad noise figure that will improve with a low noise amplifier.

The problem starts if the receiver can't handle a 20dB increase in gain, and few receivers can. You'll have to attenuate the signal down to where a scanners poor receiver can handle it. An adjustable attenuator at the scanner are the best to use and are adjusted for best signal/noise ratio while monitoring a weak signal. The more filters you can use to reduce the amount of off frequency signals the better it will be.

/Ubbe
 

n1chu

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... amplifying nothing gives you nothing. If the signal isn’t there to begin with, or severely in the weeds where the noise floor is a factor, amplifying both leaves you with a zero gain. An increased signal does no good if the noise is also amplified. First, there must be an improvement in signal strength. I’ve tried the variable adjust amps with limited success and have come to the same conclusions. An amp won’t satisfy but an antenna that has a gain factor will. I guess I’m going to build one to spec.
 

Ubbe

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An increased signal does no good if the noise is also amplified. First, there must be an improvement in signal strength.
You have to study more on internal noise figures. If the amplifier have a less noise figure than the scanner, then the signal will improve with that difference in dB. Any signal loss in the coax will also improve the signal with that amount of dB if amplification are used before the point of the signal loss.

You have to increase the signal level before the signal are burried in the internal noise of the coax and scanners front-end, or the loss of signal level due to the mismatch between the direct connection of a badly tuned antenna to a coax.

/Ubbe
 
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