multiple questions

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jhsands

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Hello all and good afternoon/morning.

First off, I have spent over an hour looking through Passive Devices and Amplifier Discussion forums.

[1] I have the radio shack pre-amp 15-2507, but obviously it is 75 ohm. Explain how that will work since the antenna is 50ohm and the LMR400 is 50ohm?

[2] One of my target towers is over my roofline. My antenna tip is currently at 128", however my roofline is 159".

Do I have to go to 256" to gain enough signal (3dB), or will just moving the antenna bottom over the roofline increase that much? Say start the bottom of the antenna at 170" so it sees over the roof.

[3] I have a 9dBD 800mhz yagi I am wanting to combine with a VHF unity gain stick. Can someone point me to the proper hardware? Is this a multiplexor, combiner, ... ?. A link would be wonderful :) Please PM me if you cannot provide the link in the forum.
 

K9WG

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Greenfield, Indiana USA
[1] I have the radio shack pre-amp 15-2507, but obviously it is 75 ohm. Explain how that will work since the antenna is 50ohm and the LMR400 is 50ohm?
Contrary to popular belief the 75/50 mismatch will not cause that much signal loss. The challenge will be adapting the 50 ohm connectors to the 75 ohm connectors.

[2] One of my target towers is over my roofline. My antenna tip is currently at 128", however my roofline is 159".

Do I have to go to 256" to gain enough signal (3dB), or will just moving the antenna bottom over the roofline increase that much? Say start the bottom of the antenna at 170" so it sees over the roof.
For best posistioning the antenna need to be clear of the roofline. This being said I would try it in the current configuration first.

[3] I have a 9dBD 800mhz yagi I am wanting to combine with a VHF unity gain stick. Can someone point me to the proper hardware? Is this a multiplexor, combiner, ... ?. A link would be wonderful :) Please PM me if you cannot provide the link in the forum.
You might try a simple splitter (type used for TV/CATV) A splitter will work as a combiner when used with two antennas. Don't be afraid to try things out. Worst case is it will not work.
 

Token

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Mojave Desert, California, USA
Hello all and good afternoon/morning.

First off, I have spent over an hour looking through Passive Devices and Amplifier Discussion forums.

[1] I have the radio shack pre-amp 15-2507, but obviously it is 75 ohm. Explain how that will work since the antenna is 50ohm and the LMR400 is 50ohm?
Knock yourself out, hook it up. The small SWR bump the mismatch will cause is nothing at all. Such an issue can be measured, to be sure, particularly in say a lab environment, but in practical application for scanner type signals it does not matter. If you were talking weak signal modes, say EME or something like that, then it would be worth the effort to use the correct pre-amp.

[2] One of my target towers is over my roofline. My antenna tip is currently at 128", however my roofline is 159".

Do I have to go to 256" to gain enough signal (3dB), or will just moving the antenna bottom over the roofline increase that much? Say start the bottom of the antenna at 170" so it sees over the roof.
Get the base of the antenna above the roof line.

Not sure why you think doubling the height of the antenna is worth 3 dB. Antenna height, by itself, really has little to do with antenna gain. Yes, I know, that is not quite right when you start talking about lobe shaping and ground gain, but for practical purposes gain is not a huge function of antenna height once you get above one wavelength off the ground. It most certainly is not a linear relationship, where twice the height is twice the gain.

What increased antenna height can do is improve line of site. If you can gain line of site to the target your potential signal increase over heavily shaded can be a whole lot more than 3 dB.

[3] I have a 9dBD 800mhz yagi I am wanting to combine with a VHF unity gain stick. Can someone point me to the proper hardware? Is this a multiplexor, combiner, ... ?. A link would be wonderful :) Please PM me if you cannot provide the link in the forum.
A power combiner or a signal splitter can do what you want it to do…sort of. And if it is not hard to set up you could probably give it a try, just because. But, this is not a recommended practice and I would not bother myself.

Because of the chaotic nature of the phase relationship of the two antennas you might end up completely killing some signals, while improving others. For example, the 800 MHz antenna is still going to receive in the VHF range. If the phase relationship between the same signal on the two antennas for any given received VHF signal approaches 180 degrees (or multiples of it) the result will be a reduction in energy at your receiver. Or the signal at the radio will be lower level than on either antenna by itself, even the 800 MHz antenna alone on a VHF signal. The VHF antenna can do the exact same thing with UHF signals.

To do it right you want to use a Duplexer (actually, this is really a diplexer, but manufacturers are marketing them as “duplexers” and “multiplexers”, to call a diplexer a duplexer is not wrong, but to call a duplexer a diplexer can be wrong...depending on the design of the duplexer). One with a LPF filter on one port, allowing VHF operation, and an HPF on the other port, allowing UHF operation. You would then put the VHF antenna feed into the VHF port and the Yagi into the UHF port. The combined port would go to the radio. The attenuation of any VHF signals being received by the UHF antenna, and UHF signals received by the VHF antenna, should make any resultant phase cancelation minimal or below significant levels. I have not seen a model that is specifically built for the band pairs you quote, but the Diamond MX37 is pretty close, and the Comet 4130B even closer yet.

T!
 
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