2 radios on 1 antenna

jmr3865 · Post by **jmr3865** » Wed Jul 27, 2005 10:18 am

1 scanner, 1 2 way radio on 1 coax to 1 antenna. Is it possible?

tvsjr · Post by **tvsjr** » Wed Jul 27, 2005 10:44 am

Maybe, with a coax relay to disconnect the scanner when the TXer goes hot. One "oops" and you let the magic smoke out of your scanner.

Another antenna and second run of feedline is likely easier.

jmr3865 · Post by **jmr3865** » Wed Jul 27, 2005 12:14 pm

what if i turn the scanner off when i tx?

phrawg · Post by **phrawg** » Wed Jul 27, 2005 12:21 pm

no difference. Still will blow up the scanner.

Being powerless does not decrease the "POOFability" of the scanner.

Phrawg

tvsjr · Post by **tvsjr** » Wed Jul 27, 2005 12:27 pm

Yep, phrawg's right. Besides, even if turning it off would save the scanner, you'd forget sooner or later!

jmr3865 · Post by **jmr3865** » Wed Jul 27, 2005 12:41 pm

What should I do besides getting a 2nd antenna?

n2knc · Post by **n2knc** » Wed Jul 27, 2005 12:56 pm

pick the radio or scanner to have the antenna on or do like evey one else says,buy another antenna and run another feedline.

kurt meltzer · Post by **kurt meltzer** » Wed Jul 27, 2005 1:47 pm

You didn't specify the frequency range of the 2-way radio, or the frequency range(s) the scanner is used for, but: If the scanner is not programmed for any frequencies that fall within the same range as the 2-way radio, then this could be accomplished with a diplexer.

Example: VHF transceiver and scanner (IF scanner is programmed ONLY with freqs in the UHF range) could share a common VHF antenna and coax (a VHF 1/4 wave mobile antenna will actually work fairly well with a UHF scanner). Just use a VHF/UHF diplexer (also known as a cross-band coupler), such as Sti-Co MM-1-150/450 or Maxrad MDF-150/450.

If the scanner is programmed with freqs that are within the same band as the 2-way radio, then this is not practical, as pointed out by others above.

Tom in D.C. · Post by **Tom in D.C.** » Wed Jul 27, 2005 2:01 pm

I believe that no matter how you slice it, putting RF at virtually ANY normal transmit level (say 1 watt and up) and frequency into a MOSFET or FET RF amplifier will fry the circuit.

The only way this will work is if you never transmit, and that is obviously not in the cards for this installation.

EKLB · Post by **EKLB** » Wed Jul 27, 2005 3:04 pm

Well for the most part id have to believe youd be best off by getting a tee adaptor for your AM/FM music radio and simply plug into it and use the vehicles factory antenae for rx on both the Music radio and scanner combined.

Although it may not be a perfect match for a freq your scanning it would rx across the band fairly well.

The majority of the time a scanner is not exact for antenae tuning as the scanned freqs vary for example from lo band up into UHF ect.

But you wont poof the scanner and would still be able to use the music radio as well.

EKLB

kurt meltzer · Post by **kurt meltzer** » Wed Jul 27, 2005 3:16 pm

It's true that any normal transmit power level applied to the RF input of a scanner (or most any receiver) will damage that receiver, however: No such harmful signal level will appear at the RF input of your scanner if used with a diplexer as I've described above.

The diplexer is a frequency selective device that seperates two bands of frequencies from each other, while allowing both to "see" the antenna.

In the VHF/UHF example I mentioned:

VHF signals applied to the VHF port of the diplexer pass through to the antenna with only nominal insertion loss (i.e.: <0.5dB).

UHF signals applied to the UHF port of the diplexer pass through to the antenna with only nominal insertion loss (i.e.: <0.5 dB).

BUT: The two ports (VHF and UHF) are isolated from each other such that neither radio sees a harmful level of RF from the other. The actual isolation is on the order of 40-50 dB (varies by manufacturer). I have personally used the Sti-Co cross band coupler to transmit on VHF at 100 watts while simultaneously receiving on UHF (and vice versa) using a single antenna, and it works fine. You can even TRANSMIT simultaneously on both radios with no ill effect, as long as your antenna is resonant on both bands.

For more info regarding the diplexer's theory of operation, do a Google search and you will find several articles there - or contact Sti-Co Industries and describe your application and ask them about their line of diplexers (they refer to them as cross-band couplers - same thing). If your scanner only needs to receive in a single band, and if that band is different than your 2-way radio's band, then you could definitely make this work with one of their products.

This technique (one antenna serving multiple radios) is very common in covert vehicle installs and is definitely worth exploring if you need a low profile installation. But it's always cheaper to just use two antennas. Seperate antennas will work a little better, too (no insertion loss).

Best of luck with your project.

kf4sqb · Post by **kf4sqb** » Wed Jul 27, 2005 3:30 pm

Just an idea, but how about PIN diodes? If the power output of the radio isn't very high, say below about 50 watts, I would think that this should work. That's what keeps the transmitted RF from feeding back into the receiver in many lower-powered two-way radios. You would probably see some degradation of scanner performance, as I am sure the diodes would introduce some loss into the system, but you should be able to overcome this with a pre-amp. Thoughts?

jmr3865 · Post by **jmr3865** » Wed Jul 27, 2005 4:51 pm

thank you, so bottom line, i need another antenna.

thebigphish · Post by **thebigphish** » Wed Jul 27, 2005 5:18 pm

in a nutshell, yes

wa2zdy · Post by **wa2zdy** » Thu Jul 28, 2005 4:50 am

EKLB wrote:Well for the most part id have to believe youd be best off by getting a tee adaptor for your AM/FM music radio and simply plug into it and use the vehicles factory antenae for rx on both the Music radio and scanner combined.

Although it may not be a perfect match for a freq your scanning it would rx across the band fairly well.

The AM receiver will usually pull the system down so the scanner sees almost a short across it's antenna input. Obviously the AM/FM receiver is designed with this in mind; the scanner is not.

One would need the appropriate coupler for this to work.

jmr3865 · Post by **jmr3865** » Thu Jul 28, 2005 6:25 am

would it work If I was not transmitting?

440roadrunner · Post by **440roadrunner** » Thu Jul 28, 2005 8:13 am

Even if you don't transmit, if you simply "tee" two receivers/ scanners together, you may get into operational trouble.

Many scanners have "dirty" local osciallators, and either radio may generate enough local energy from the mixing oscillators to interfere (on some frequency(s) with the opposite receiver.

Not only that but you WILL loose some sensitivity due to mismatch. I would think this could get complicated. If you hook two 50 ohm dummy loads to a tee, you've just created a 25- to 50 ohm mismatch, but in this case, you've got the interconnecting cables---which depending on their lengths, will act as transformers. Seems to me you could create a situation where one cable is "just the right length" accidently to cause tremendous attenuation

Receive only:

Minimum you need a "splitter" sometimes called a "combiner" You will lose 3dB to each device.

One on VHF, other on UHF, you could also use a diplexer as discussed above.

jmr3865 · Post by **jmr3865** » Fri Jul 29, 2005 1:16 pm

ok, well I semi put it on today. I didn't install it on the chimey but did hold it up. I put it on the scanner. I got more than I wanted to get, mobile it is than from now on.

raymond345 · Post by **raymond345** » Fri Jul 29, 2005 11:06 pm

if you install an antenna switch and a dummy load on
the switch when the tx radio is not using the antenna
it will work great and it is super cheap to get these
parts used on my sites like http://www.ebay.com

Most ham operators run many radios on one antenna
all the time.We also have one antenna switch port
that is grounded out.When not using the equipment
the antenna goes to ground.That will SAVE a lot of money.

nmfire10 · Post by **nmfire10** » Sat Jul 30, 2005 6:17 am

Yea, simply putting a splitter on it and connecting even two receivers will not work as well as it looks on paper. I tried this in my house and still have it hooked up as the loss isn't killing it completely. I have the antenna on the roof split going to the scanner on the desk and my pager's amplied charger antenna jack. There is a HUGE absolutely noticiable difference as soon as you connect the second device. I tested this for example, with a NOAA channel on the scanner. It comes in full quieting. I plug the pager base in and it sounds like someone moved the WX transmitter another 50 miles away.

Cowthief · Post by **Cowthief** » Mon Aug 01, 2005 12:04 pm

Hello.

What will work, and work well.
Get an RF preamp/multi-coupler for the AM/FM whip.
The units made by A/S work quite well, as there is even something of a tuned circuit for both the AM and the FM side.
75 ohms for the broadcast radio and 50 ohms for the scanner.
As it is a buffer amp, it will get rid of "birdies" that 2 radios connected together can cause, and it may even improve your FM reception.
They are around $25 or so last time I looked.

n9upc · Post by **n9upc** » Mon Aug 01, 2005 2:02 pm

OK folks read the posts it is not going on a car it is going on a house from what is being typed.

Go and look at either getting a coax switch, or get a power divider/splitter.

Many PS agencies around her use this to be able to run 2, 3, or 4 spectra consolettes on 1 800Mhz antenna.

To do what you want to do is going to be a little tough with a scanner and a two-way. The power divider/splitter is usually located to a certain freq range. (aka 150 - 160Mhz, 450 - 470Mhz)

This may be a problem if you have a VHF mobile and are scanning some other non-VHF freqs using a power divider/splitter.

If I can find the part number and you can tell me what band you are using on the two way I can look-up the part number for you.

MITYTONE · Post by **MITYTONE** » Wed Aug 10, 2005 4:55 am

If the 2 radios (Tx/Rx) are minimum 5 mhz apart, with a properly tuned duplexer.That should work fine..

2 radios on 1 antenna

2 radios on 1 antenna

1 antenna + 1 scanner + 1 transmitter = BLAM!

yes you can use two radios /cheap

What.

2 radios 1 antenna