Omni Yagi

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scanmanmi

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I like having an onmi on vhf but am having trouble picking up some 800 towers. Is there some info about maybe using more than one antenna on the same wire? Has anyone done this with success? Is phasing an issue?
 

jonwienke

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A cable TV splitter can be used as a combiner if you connect it "backwards"--individual antennas to the outputs and the main antenna coax to the input. Good ones will give you 20dB or more of isolation.
 

prcguy

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TV splitters at best have about 4dB loss per leg and some are over 5dB, I've measured lots of them with 75 ohm vector network analyzers and the specs are pretty grim. Using them to combine two exact antennas in phase with identical length coax will result in just a little improvement due to the poor performance of TV splitters. If you try to combine two different antennas with a TV splitter and random lengths of coax you will have varying results from maybe a dB or two improvement if the antennas, spacing, the moon and the planets are all aligned, to many dB of loss when they are not.

If you are just combining a VHF or HF through 220MHz antenna and another for 800MHz you can use a Comet CF-416W diplexer, which passes 1.3 to 250MHz on one port and 400 to 1400MHz on the other port. There is also the Diamond MX-37 diplexer which now shows 1.6 to 470MHz and 900 to 1300MHz but older versions showed something like 840 to 1300MHz.

Either of the diplexers will have very minimal loss combining two antennas of different frequency ranges.
prcguy

A cable TV splitter can be used as a combiner if you connect it "backwards"--individual antennas to the outputs and the main antenna coax to the input. Good ones will give you 20dB or more of isolation.
 

buddrousa

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It is back to using equipment designed for the purpose it is being used for.
 

jonwienke

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I agree that there are a lot of crappy splitters out there. But they are your only option if you want to run two antennas in the same frequency range.
 

cpetraglia

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You can run two feeds in to a quality antenna switcher. You just have to remember to use it when changing bands. It is a lot more cable, but enables a dedicated antenna of choice.
 

buddrousa

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The correct way is to take the ham diplexor tower top mount it connect the 2 antennas then use the line coming down the tower to run to the scanner this is what these devices were made for works with both RX and Tx up to the rated power listed on the diplexor.
 

scanmanmi

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I don't want to transmit just receive so I don't need a duplexer. I am just wondering about receiving pattern of the combined antennas. If I get a high gain 800 yagi and parallel a vhf onmi with it will it affect the gain of the yagi?
 

buddrousa

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No but you need the ISOLATION that the ham DIPLEXER gives you so the 2 antennas do not interfere with each other electronically
 

buddrousa

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There are combiners at the top of our 750 foot tower for the receive antennas for each band that runs up the wave guide same for the transmit side antennas we are combining 800 460 and 150 on the split antennas for receive and transmit.
 

prcguy

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You don't need a diplexer just for receive. That is if you can live with loosing more than half your signal levels in a splitter used backwards as a combiner.

If you want to preserve the signal levels from your antenna you have to use a diplexer to combine two antennas of different frequency ranges and a diplexer will do this with just a fraction of a dB loss. You can run your 800 Yagi and a VHF omni or a VHF/UHF Discone or some other combination and they will all play together nice.
prcguy


I don't want to transmit just receive so I don't need a duplexer. I am just wondering about receiving pattern of the combined antennas. If I get a high gain 800 yagi and parallel a vhf onmi with it will it affect the gain of the yagi?
 

lmrtek

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I like having an onmi on vhf but am having trouble picking up some 800 towers. Is there some info about maybe using more than one antenna on the same wire? Has anyone done this with success? Is phasing an issue?
Again, 75 ohm equipment makes it easy

Buy a RCA vhf/UHF tv preamp for 25 bucks

It has separate vhf and uhf inputs so it can easily combine a vhf omni and a 800mhz antenna and will eliminate all feedline loss so it will outperform any passive device like splitters, combiners, and diplexers
 

prcguy

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Many TV amps with separate VHF/UHF inputs will roll off frequencies below 50MHz leaving out the VHF low band, plus they will roll off between 88 and 172MHz then roll off between 220 and 470MHz. Your reception of the entire VHF and UHF public service bands would be severely attenuated. They do this to protect the preamp from overloading from FM broadcast and nearby public service transmissions. Some don't do this but unless you specifically know the model and its specs it could.

Then there are the CATV amps with built in splitters, these were designed to be used in a closed cable TV system with signal levels within very prescribed limits. Many of them will be completely overloaded by off air signals well above what they were designed for.

Some CATV amps are better than others and some people live in areas with little RF activity to overload these amps. For those that can actually use a CATV amp without all the problems I listed, consider yourself lucky. Where I live you can't use them and I have not found a single brand or model that does not create lots of intermod and in general cause a reduction in reception.

One size does not fit all when you are recommending an amplifier or amplified splitter to use on an antenna.
prcguy


Again, 75 ohm equipment makes it easy

Buy a RCA vhf/UHF tv preamp for 25 bucks

It has separate vhf and uhf inputs so it can easily combine a vhf omni and a 800mhz antenna and will eliminate all feedline loss so it will outperform any passive device like splitters, combiners, and diplexers
 

N9PBD

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Although prcguy mentioned two antennas with a properly phased harness, and some opportunity for gain, what nobody has mentioned is what happens when you don't take the phase, or time of arrival into account. When two antennas, separated by some unknown multiple of wavelengths, receive the same signal, at roughly the same time, but slightly out of phase, you can actually cancel out the intended signal on your coax. Many of you have already experienced this with a single antenna mobile installation, with what many call "picket fencing". This is caused by the signal having more than one path to your antenna, and the signals from those multiple paths arriving slightly (or maybe 180 degrees) out of phase, and causing cancellation or nulling. The same thing can occur with two antennas feeding the same feedline. I'm not sure what the results of an omni antenna and a directional yagi would be, but you're better off with maybe separate antennas and feedlines, and using a coaxial switch to select the one you want connected to your receiver.

This subject comes up from time to time, and many people gave some good inputs here: http://forums.radioreference.com/splitters-filters-multicouplers/113057-running-one-scanner-multiple-antennas.html
 

prcguy

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I hinted at that in post #4 but thanks for your more complete explanation.
prcguy


Although prcguy mentioned two antennas with a properly phased harness, and some opportunity for gain, what nobody has mentioned is what happens when you don't take the phase, or time of arrival into account. When two antennas, separated by some unknown multiple of wavelengths, receive the same signal, at roughly the same time, but slightly out of phase, you can actually cancel out the intended signal on your coax. Many of you have already experienced this with a single antenna mobile installation, with what many call "picket fencing". This is caused by the signal having more than one path to your antenna, and the signals from those multiple paths arriving slightly (or maybe 180 degrees) out of phase, and causing cancellation or nulling. The same thing can occur with two antennas feeding the same feedline. I'm not sure what the results of an omni antenna and a directional yagi would be, but you're better off with maybe separate antennas and feedlines, and using a coaxial switch to select the one you want connected to your receiver.

This subject comes up from time to time, and many people gave some good inputs here: http://forums.radioreference.com/splitters-filters-multicouplers/113057-running-one-scanner-multiple-antennas.html
 

buddrousa

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Using a TV splitter and TV amp is like using a BATHTUB for a boat to fish out of. If you stop up the drain it will float but a fishing boat works much better. People come here ask questions get answers from people that work in the field or have worked in the field. They get the parts that are designed for the use intended and have tested factory designed parts against cheap parts not designed for the intended use. The short answer is yes it may work but like I said so will a bath tub work as a fishing boat.
 
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