wish to combine VHF rf tx signals into one antenna

[Chip Fetrow]

I believe we are discussing apples vs. stove bolts.

The original message was regarding using several VHF radios at the  
same time.

Yes, this is possible, but it requires two antennas, one for all  
transmitters, and one for all receivers.  On receive, there is  
basically no loss.  On transmit, there is loss.  It starts when the  
transmitters are about 600-800 kHz apart and climbs to a bit over 6 dB  
as the transmitters reach the same frequency.  Yes, it is possible to  
combine two transmitters on the same frequency.  If they just run, you  
get 6 (plus a little) dB of loss.  If they are on exactly the same  
frequency, fed in phase, and modulated exactly the same, you actually  
get 3 dB of gain.  Broadcast transmitters do this all the time.  It  
used to be two VHF transmitters, and maybe up to 5 UHF TV transmitters  
(back in the day of 6 MW NTSC ERPs).  Today, solid state broadcast  
transmitters can combine dozens of PA modules, even on MW.

Believe it or not, by using the PROPER receive antennas, it is  
possible to transmit and receive on the same frequency, at the same  
site at the same time.

I have a LOT of experience building combiners and combined sites.  We  
were having so much success several of the Engineers at Sinclair Labs  
wanted to test one of our sites.  We felt we were spending more than  
needed on cavities -- and we were right.  We had two VHF Sinclair  
235-2 (VHF, 6 dB gain, omni) antennas. one at the top of a pole on a  
office building, and one 40 feet below it, exactly below the receive  
antenna.

We told all the tenants we were gong to take the system down at 6 AM  
on a Saturday morning and we disconnected the VHF transmit combiner  
and the VHF receive multicoupler.  The transmission lines had cross  
band couplers for low-band VHF, UHF, and 900 MHz transmitters, and UHF  
and 900 MHz receivers. We had no low-band receivers, only paging  
transmitters.

We hooked up my IFR Com 120B and put a signal into the transmit  
antenna, and looked for the receive signal on the receive antenna.   
The Com 120B has both a meter and a spectrum analyzer.  No joy.  We  
disconnected the Com 120B from the transmit antenna and connected an  
IFR 1200, then an IFR 500.  No joy.  We just couldn't find the  
transmitted signal, but it is only about 1 Watt.  I kept saying we  
should connect the ambulance company's transmitter, which is 110 Watts  
and try to find it.  I kept being told no.  Finally, when everyone was  
as drained as much as I was, they agreed.  (We didn't want to fool  
with the Secret Service radios, or the FCC radio -- a rare VHF  
repeater (in commercial use), or the FAA radio.

We connected the ambulance radio and keyed it up.  Still, no joy.  We  
only expected 80 dB or so of isolation.  We were showing it was much  
more than 110 dB.  IMPOSSIBLE!  Well, as we were staring at the test  
equipment, wondering what we were doing wrong, an ambulance called in  
from 35 miles away.  It had a 35 Watt radio and a 3 dB gain antenna.   
So, we were transmitting 440 Watts ERP, from 40 feet away, and we were  
hearing a 70 Watt ERP radio from 35 miles away!  Isolation works!  We  
were all dumbfounded, none as much as the Sinclair Labs Engineers.   
This means a simplex, or single frequency real-time repeater is  
possible.


  Zach (good to see you on here) wrote about mobile duplexing.  I used  
to do that all the time.  I had three VHF high-band radios in the  
trunk of my Maserati.  I had two in my Blazer.  I could use the auto- 
patch on our repeater in full duplex, that is to write, I could hear  
myself coming back, and the person on the phone could interrupt me.   
It was mostly like actually talking on the phone.  600 kHz is pretty  
far when using separate antennas, even at close spacing.  On a single  
antenna -- not so much.  You will burn out the receiver if you just  
use a RF T connector.


What Sandy was writing about is cross-band coupling.  That is, using  
the same transmission line for different bands.  It works well.  Hams  
use multi-band antennas, but the commercial world uses separate  
antennas, so the bands are split apart at the top of the tower or pole.

The ham antennas work OK, but their vertical elevation patterns are  
poor.  They shoot far too much signal at a high angle, and they have  
minor lobes that go down, which will cause self interference at a  
combined use site.

Commercial "fiberglass" antennas have the same problem.  The problem  
is how they have to be constructed.  The velocity factor of the  
transmission line is less than one, and since those segments are also  
the radiators, the phase lags as the RF gets to the elements further  
from the feed point.  Thus, the elevation pattern sucks.

The only antennas that can be fed in phase are external dipoles, or in  
most cases, external loop dipoles are used.  They have great elevation  
patterns, and as long as one doesn't try for more than 6 dB of gain,  
there are few minor lobes, or none for a 3 dB antenna.  Those minor  
lobes will kill you on both isolation, but also in multi-path  
interference in the field.  I will never understand why hams, and some  
commercial operators insist on 10 dB gain antennas.  It gains the user  
nothing, and makes the resulting coverage worse.  Sure, you have a  
higher ERP, but it shoots over everyone's heads.


Back to the original question -- three of four VHF high-band radios  
into one antenna -- well, it IS possible, but you don't want either  
the restrictions nor do you want to pay the freight.  You REALLY MUST  
HAVE TWO ANTENNAS, one for receive and one (or two) for transmit.  Why  
would you want two for transmit?  As I wrote earlier, as the  
frequencies get close, the losses go up.  If you can place those close  
transmitters on separate antennas, you can avoid those big losses.

If you really need to use only one antenna, you are forced into a  
position where you are going to have a port for each frequency.  So if  
you want to use two repeaters, that isn't that bad, maybe $3000.  If  
you want four, it will be maybe $8000.  If you want 10, well, that  
would be just nutz and you would save a lot of money by going to two  
antennas.


Regarding one antenna per transmitter, that has its own set of  
problems.  The transmitters are dong to desense the receivers.  You  
really need to consider two antennas, one mounted above the other.   
You can use parallel combiners, which have a loss of at least 3 dB per  
transmitter, but you can have the freedom of transmitting on any  
frequency in the band.


One last thing.  ANY of these solutions, except for one antenna per  
radio, is going to require modifying the radios.  You are going to  
have to put an additional antenna jack on the transmit side.  You  
cannot use the T/R switching inside the radio.  You still will likely  
want to have the T/R switch in the radio, but use it to just short the  
receive antenna input to ground.  That may not be needed, but since it  
is already in the radio, why not use it?

--chip

On May 30, 2013, at 1:00 PM, tacos-request at amrad.org wrote:

> Message: 5
> Date: Thu, 30 May 2013 09:54:14 -0400
> From: Zachary Yarashus <KJ4BXT at ARRL.NET>
> To: tacos at amrad.org
> Subject: Re: wish to combine VHF rf tx signals into one antenna
>
> Actually, the duplexer might work better than that... I saw a mobile  
> setup
> demonstrated in which there was a single antenna, and two  
> transceivers. One
> radio could transmit without observably diminishing the receive  
> performance
> of the other radio, and vice versa. They could also transmit or
> receive simultaneously. Though in general they might not need to do  
> that, I
> thought it was rather impressive.
>
> By the way, is it duplexer or diplexer? I've generally heard  
> duplexer, but
> are these just two different names for the same thing?
>
> On Wed, May 29, 2013 at 4:28 PM, Nan and Sandy Sanders <radiodog77 at pobox.com
>> wrote:
>
>> You can combine by band. 1 6M, 1 2M and 1 70CM radio into your  
>> triple band
>> antenna. Other radios can be connected to other antennas 1 per band.
>> However......... as a practical matter you will only be able to use  
>> one
>> radio per band at a time because while you are transmitting any  
>> radios
>> receiving on the same band will be desensitized and will hear only  
>> very
>> strong signals if any signal at all. If you are at Tacos Sat we can  
>> discuss
>> this a bit more.
>>     Sandy
>>     WB5MMB
>>
>> At 03:13 PM 5/29/2013, you wrote:
>>
>>> Hi, this might be odd. At K3GWU, as we catalog all donated  
>>> equipment, I
>>> have (now) several VHF radios. Some are portable (1-5W), others  
>>> fixed, low
>>> power (10W), medium power (30W), high power (>60W)
>>>
>>> Is there a practical way to combine these into one antenna? or  
>>> should I
>>> use different antennas?
>>>
>>> I am seeking to minimize cabling from 3rd floor to rooftop (+2  
>>> floors),
>>> and to allow the station to be constantly in service with voice to  
>>> any
>>> repeater network in line of sight with Foggy Bottom, and some  
>>> experimental
>>> support in VHF band with digital modes.
>>>
>>> Ideas would be appreciated. I noticed the availability of  
>>> diplexers, but
>>> they are not usually VHF1, VHF2 VHF3 > common outputs.
>>>
>>> I do have a V2000A diamond triple band antenna (6M/2M/70CM) and a
>>> separate KA2ZEV 2M antenna now that I could use....
>>>
>>> 73 de N3RDX