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N6DDD
11-03-2005, 09:41 PM
Hello,

It was suggested to me that I should just use one feedline for both antennas as the signal will be drawn to the best matched outlet so 2 meters will not go through the 70cm antenna and vice versa. It makes sense if you think about it, but I need more convincing before trying it.

Has anyone experimented with this, and what were the results? Would the signal strength be the same as if using seperate feeds?

Best regards,

Arlen KI6BCC

WB2WIK
11-03-2005, 10:00 PM
Whoever gave you that advice is making me jealous because I wish he'd share the drugs.

That absolutely, positively does not work.

You can, however, use a single transmission line for two antennas for different bands if you use a "duplexer," which is a device that is a TX/RX signal splitter containing internal filters to separate VHF from UHF signals. Those are commonly available almost anywhere that sells antennas, but they can easily cost as much as a second feedline, so balance the cost vs. benefits and make the decision.

WB2WIK/6

KL7AJ
11-03-2005, 10:09 PM
Quote[/b] (KI6BCC @ Nov. 03 2005,14:41)]Hello,

It was suggested to me that I should just use one feedline for both antennas as the signal will be drawn to the best matched outlet so 2 meters will not go through the 70cm antenna and vice versa. #It makes sense if you think about it, but I need more convincing before trying it.

Has anyone experimented with this, and what were the results? #Would the signal strength be the same as if using seperate feeds?

Best regards,

Arlen #KI6BCC
That only works for HF antennas that are actually fed at the same point (usually at right angles.) It doesn't work if you have different runs of coax between them...unless your coax lengths are chosen very carefully, under which condition you're created a poor-man's duplexer/diplexer anyway. You can't just randomly run them in parallel....unless you're REALLY lucky!

eric

WB2WIK
11-03-2005, 10:36 PM
Quote[/b] (kl7aj @ Nov. 03 2005,15:09)]
Quote[/b] (KI6BCC @ Nov. 03 2005,14:41)]Hello,

It was suggested to me that I should just use one feedline for both antennas as the signal will be drawn to the best matched outlet so 2 meters will not go through the 70cm antenna and vice versa. #It makes sense if you think about it, but I need more convincing before trying it.

Has anyone experimented with this, and what were the results? #Would the signal strength be the same as if using seperate feeds?

Best regards,

Arlen #KI6BCC
That only works for HF antennas that are actually fed at the same point (usually at right angles.) #It doesn't work if you have different runs of coax between them...unless your coax lengths are chosen very carefully, under which condition you're created a poor-man's duplexer/diplexer anyway. #You can't just randomly run them in parallel....unless you're REALLY lucky!

eric
...and nobody'd be that lucky for a couple of reasons.

The primary one is that 2m and 70cm are odd harmonically related, so the 2m antenna is also resonant at 70cm. As such, the majority of 2m antennas look like 50 Ohms at their third harmonic (70cm), and when you place two in parallel, if they're exactly in phase, you have now loaded your transmission line with 25 Ohms: Two 50 Ohm branches in parallel. That's not good.

Besides the mismatch, both will radiate at 70cm, and that's also not good.

At some frequencies in the 70cm band, the two antenna radiation patterns are almost guaranteed to phase cancel each other and create a mess.

This is just a seriously bad idea...

WB2WIK/6

KA3RFE
11-03-2005, 10:53 PM
Whoever told you this is an idiot. The signal will travel to BOTH antennas and the radio not being used will have strong RF currents being fed to it which can damage the radio. You would have spurious emmssions for sure on UHF and the SWR on either radio would probably be too high also.

Whether you could use a duplexer on the coax, I don't know. I have used duplexers that hook two radios on two different bands to one multiband antenna, but not two radios to two antennas on the same feed line.

Perhaps Glenn knows if duplexers can be used this way.

WB2WIK
11-03-2005, 11:01 PM
Quote[/b] (KA3RFE @ Nov. 03 2005,15:53)]Whoever told you this is an idiot. The signal will travel to BOTH antennas and the radio not being used will have strong RF currents being fed to it which can damage the radio.

Whether you could use a duplexer on the coax, I don't know. I have used duplexers that hook two radios on two different bands to one multiband antenna, but not the other way around. And I do not know if putting it on the line backwards would even work. I wouldn't not try to do that myself, I'd rather not experiment with my radios that way.

Perhaps Glenn knows.
Glen knows, and so do I, and so do lots of people.

Duplexers are passive filtering devices that are bi-directional. You can use one rig with two antennas, or one antenna with two rigs, it doesn't matter.

What does matter are the cutoff frequencies and power handling rating.

WB2WIK/6

KB7UXE
11-03-2005, 11:14 PM
look up "Occams Razor"...

( use seprate coax)

KA3RFE
11-04-2005, 12:09 AM
Quote[/b] (WB2WIK @ Nov. 03 2005,16:01)]
Quote[/b] (KA3RFE @ Nov. 03 2005,15:53)]Whoever told you this is an idiot. The signal will travel to BOTH antennas and the radio not being used will have strong RF currents being fed to it which can damage the radio.

Whether you could use a duplexer on the coax, I don't know. I have used duplexers that hook two radios on two different bands to one multiband antenna, but not the other way around. And I do not know if putting it on the line backwards would even work. I wouldn't not try to do that myself, I'd rather not experiment with my radios that way.

Perhaps Glenn knows.
Glen knows, and so do I, and so do lots of people.

Duplexers are passive filtering devices that are bi-directional. #You can use one rig with two antennas, or one antenna with two rigs, it doesn't matter.

What does matter are the cutoff frequencies and power handling rating.

WB2WIK/6
OK so using a duplexer "backwards" will work ok. I did not think that was possible. I've always considered duplexers to be two radios to one antenna/feedline. I am still leeary about the RF fields from the antennas getting into each other. You seem to be saying that isn't a problem either?

OK I learned something new. http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif

WB2WIK
11-04-2005, 12:12 AM
First, "RF fields from antennas getting into each other" doesn't matter a bit. Go to any mountaintop transmitter site and you'll find hundreds of antennas on towers all right next to each other, often times with 20 or 30 antennas on a single tower, and a lot of these are broadcast stations running 50 kilowatts at the same time. So what?

Second, the purpose of the duplexer is to separate the VHF and UHF signals, so when you transmit on VHF, it routes all your power to only the VHF antenna; when you transmit on UHF, it routes all your power only to the UHF antenna. Thus, in this situation, only one antenna radiates, anyway.

WB2WIK/6

N6DDD
11-04-2005, 12:49 AM
Thanks, all!

I forgot to mention that only one radio would be connected. I wish I had another! http://www.qrz.com/iB_html/non-cgi/emoticons/sad.gif
I just couldn't see trying to save $30 or so on coax (LMR 400)
and chance having problems, yet I was curious if it would work.

Thanks again

Arlen KI6BCC

WB7DMX
11-04-2005, 01:19 AM
I have several dual band antennas, both dual band beams and vertical, they all use one transmission line, its called a phasing harnes, been useing them for many years.
the cushcraft A27010s is a dual band 5 element beam for 440 and 2 meters, it is two seperate 5 element antennas on the same boom each coax goes to a t connector for one coax down to the dual band radio transicver. works just fine and a good 1.1-1 match on each antenna