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Tieing 2 antenna together?

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KJ4FWA

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Is it possible/Wise to "Tie" two antenna together in the cable Via a splitter into one cable. Would they work together? or would this be more problem then it's worth?
 

zz0468

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chedderhead said:
Is it possible/Wise to "Tie" two antenna together in the cable Via a splitter into one cable. Would they work together? or would this be more problem then it's worth?
It really depends on how you go about it. Using an antenna diplexer designed for two or more different bands, it can work ok. Just tying multiple antennas together with a simple splitter or a tee can have unpredictable results. The antenna spacing becomes critical, as does the lenght of the cables between the antennas and the splitter or tee.
 

af5rn

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Okay, let's complicate the issue a little.

I want to run scanners off a combination of a dual-band UHF/VHF antenna and an 800 mHz gain antenna, just like most people are trying to do.

But here's the twist, I also want to simultaneously run my dual-band ham rig off the same dual-band antenna.

For this set-up, I need more than just a diplexer. I also need something below the dual-band antenna to split the signals between the diplexer and the UHF/VHF duplexer, that won't allow my transmitted signals to feed back down to the scanners.

What device would do that? Another duplexer? I'm perplexed.
 

Lowa2

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just put an isoator on the 800MHz coax. Using the 800 MHz antenna on port 1, port 2 on the coax going down to your radios.



Then use a 800MHz bandpass filter to isolate your HAM gear from your 800MHz Rx. Make sure the BP filter has enough attenuation to completely kill the ham gear from going into the Rx.

just a quick and dirty pic of what you should do:


All those components are pretty expensive, but would do a pretty good job if all the cables are phased correctly.

All that being said, if you would run 2 coax cables up there, it will most likely be cheaper, more reliable, and would work much better.
 

af5rn

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Lowa2 said:
All that being said, if you would run 2 coax cables up there, it will most likely be cheaper, more reliable, and would work much better.
Definitely! But I want BOTH antenna signals going to the receiver, not just the 800. Otherwise, I would simply run two separate cables to two separate radios.

In other words (sorry, no sweet illustrator program like yours!):
  • UHF/VHF Antenna feeds both the ham dual-bander and the scanner

    800mHz antenna feeds the scanner only
Of course, all this is based upon the theory that the two professional gain antennas from the different bands, phased together, would produce better performance than a single "all-band" scanner antenna. It is, of course, just a theory.
 

zz0468

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Lowa2 said:
just put an isoator on the 800MHz coax. Using the 800 MHz antenna on port 1, port 2 on the coax going down to your radios.

http://www.e-meca.com/specs_iso_cir/specs_iso_cir.php?ID=10&SpecsID=5All those components are pretty expensive, but would do a pretty good job if all the cables are phased correctly.

All that being said, if you would run 2 coax cables up there, it will most likely be cheaper, more reliable, and would work much better.
I see a number of flaws with your system. First, that 800 MHz filter is going to present a severe mismatch to the amateur radio. Since at frequencies other than 800 MHz, the filter is not a 50 ohm impedance, the first splitter will not provide it's specified isolation between ports, nor will it provide the specified impedance match.

The second major flaw in your design is the circulator. What may behave as a circulator at 800 MHz may look like a dead short at vhf/uhf. While circulators can be rather broadbanded, finding one that would actually provide the proper isolation on one band, while providing the proper low loss transmission on another band is goint to be difficult at best. And when is the last time you bought one that meets those specs? The one in your link certainly wouldn't cut it. The cost of the circulator alone would greatly exceed the cost of a second antenna and feedline.

The third major flaw is the requirement of critical length cables. Remember, the tranceiver is a dual bander, complicating the critical length.

I hate to say it but, while I see where you're trying to go with this, in reality, it wouldn't work as designed, and the the components required would be prohibitively expensive, that is, if you could find them at all. I'm in 100% agreement with your final comment that two antennas would be cheaper and more reliable.

To af5rn, what you're trying to accomplish would be best handled with a wideband antenna for the scanner, and a dual band antenna for the ham rig. But I've developed some ideas on running several radios on a single antenna involving coaxial relays that avoids critical length cables and expensive exotic devices such as very broadband high power circulators.

A commercial tranceiver combiner to allow several radios to share a single radio costs several thousand dollars, and is useable for a single band only.
 

af5rn

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Yeah, I was kind of afraid of that. The original theory was that running an all band scanner off of a 3db 800 antenna and a 3db dual-band antenna phased together could result in better reception than a single unity-gain tri-band antenna, like the BMAXSCAN1000 or the Spectra. Still makes sense, but is looking less and less practical.

Thanks for the input!
 

Lowa2

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zz0468 said:
I see a number of flaws with your system. First, that 800 MHz filter is going to present a severe mismatch to the amateur radio. Since at frequencies other than 800 MHz, the filter is not a 50 ohm impedance, the first splitter will not provide it's specified isolation between ports, nor will it provide the specified impedance match.

The second major flaw in your design is the circulator. What may behave as a circulator at 800 MHz may look like a dead short at vhf/uhf. While circulators can be rather broadbanded, finding one that would actually provide the proper isolation on one band, while providing the proper low loss transmission on another band is goint to be difficult at best. And when is the last time you bought one that meets those specs? The one in your link certainly wouldn't cut it. The cost of the circulator alone would greatly exceed the cost of a second antenna and feedline.

The third major flaw is the requirement of critical length cables. Remember, the tranceiver is a dual bander, complicating the critical length.

I hate to say it but, while I see where you're trying to go with this, in reality, it wouldn't work as designed, and the the components required would be prohibitively expensive, that is, if you could find them at all. I'm in 100% agreement with your final comment that two antennas would be cheaper and more reliable.

To af5rn, what you're trying to accomplish would be best handled with a wideband antenna for the scanner, and a dual band antenna for the ham rig. But I've developed some ideas on running several radios on a single antenna involving coaxial relays that avoids critical length cables and expensive exotic devices such as very broadband high power circulators.

A commercial tranceiver combiner to allow several radios to share a single radio costs several thousand dollars, and is useable for a single band only.
Don't worry, I know that most of what I said was flawed, but i missunderstood what the op wanted to do...the one thing that is screwing up his setup is the dual band VHF/UHF setup. It's hard to "phase" that. And like I said, circulators to do the kind of job he wants to do would be extremely expensive.

And it was 2am ;-) when i posted that, having just worked a 60 hr week! LOL

My next suggestion would have been coaxial relays, those are pretty darn cheap, and work pretty darn well. Ohh...and I guess i'm used to working with exotic parts, since I work on Air Navigation equipment ;-).

Cheers.
 
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zz0468

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af5rn said:
Yeah, I was kind of afraid of that. The original theory was that running an all band scanner off of a 3db 800 antenna and a 3db dual-band antenna phased together could result in better reception than a single unity-gain tri-band antenna, like the BMAXSCAN1000 or the Spectra. Still makes sense, but is looking less and less practical.

Thanks for the input!
You're better off with a wideband antenna like a discone. If you're not getting enough signal, and a good quality low noise preamp. And forget trying to diplex it with a ham rig. It's just not worth the time and trouble.
 

zz0468

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Lowa2 said:
Don't worry, I know that most of what I said was flawed, but i missunderstood what the op wanted to do...
Well, what the OP wanted, and what AF5RN wanted are two completely separate things. In the end, it all boils down to a complicated way of using a single feeline with multiple antennas and multiple radios, and is more trouble than it's worth. It's DO-ABLE, but we would have to ask why?

I have a case where i have a rather expensive high quality discone, that I may want to share with a couple of radios and a scanner. After looking at it, I came up with a scheme using a few relays and a TR sequencer to delay PTT on the radios to give the relays time to switch, then decided against it. It would have worked well, but the relays are harder to get and more expensive than another antenna and hard line. Ah, well... it was an interesting mental exercise figuring out how to do it.
 
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