My replies are in line with information you have offered in order to help me in keeping my responses organized.

On Friday, March 13, 2015 at 10:13:04 PM UTC-4, Channel Jumper wrote:
Tom W3TDH;836715 Wrote:
I am looking for recommendations for a dual band antenna that will serve
two separate radios. The reason that I want to use a single antenna is
that I have a limited number of mounting points for antennas. At
present it will be hard for SWMBO; as in Rumpole's spouse, She Who Must
Be Obeyed; to tolerate the use of both our home's gable ends and the
chimney being used to support antennas. I am planning to replace my
Diamond X-30 with a triband vertical for Six, Two, and .7 Meters. The
chimney will then support a rotor aimed Two and .7 Meter beam. The
second gable end will support the Two meter / Seventy Centimeter dual
band vertical that I am asking for help in selecting.

One radio will be a two meter packet node which will be used as a
Winlink Radio Message Server. The other radio will be a UHF D-STAR
hotspot. I am willing to pay what is needed to to get the best antenna
for this application but I don't want to waste money ineffectively. So
the two meter radio will be in the 144 MHz portion of the band and I
don't yet actually know were the D-STAR hotspot will be run. I have a
DCI filter and diplexer to keep the two radios from actually knowing of
each others existence. Since a hotspot is not supposed to be a terribly
wide area installation I would imagine that I do not want an extremely
high gain antenna but I am perfectly open to be reeducated on that. The
difficulty is that I would guess that the Radio Message Server / Packet
would benefit from as much horizontal gain as can be achieved. I have a
home brewed collinear two meter J-Pole that has been a good performer on
two meters and presents a low SWR on 440 MHz. I have yet to master
antenna modeling but I would imagine; given all the warnings I have read
on line; that it has poor radiation pattern on UHF. Is it likely to be
too poor a performer for a hot spot on UHF?

I really am asking because I want to know. I am not looking for
encouragement to do something that will be ineffective. Thank you in
advance for any help you may be willing to offer.

--
Tom Horne W3TDH

Tom,
You have too many of the same band antenna's concentrated in one place.
No matter how hard you try, they aren't going to play nice with each
other.
I read 14 posts and no one gave an even close right answer to your
question.
The answer is that you need to construct a tower, no less than 100',
near your house in order to even try to do what you wish to do.
Even then, you will need feet of separation vertically in order to get
the antenna's to play nice with each other.

Would you please name a figure for the required vertical separation? If that figure is less than Thirty Feet then could you explain why one antenna at my home's ridge line and one twenty feet higher wouldn't be enough?

I have worked net control for the Marine Corps Marathon in Washington DC for several years now and we have not had problems running two very active Two Meter Nets with only ten feet of vertical separation between the two antennas on a Fifty Foot Tall AB-952/GRC-103 self jacking mast. We also had a receive only APRS receiver set up and had no missed packets when compared with WWW.APRS.FI. The APRS is down at 146.390 MHz and thus Two MHz away from the repeater pairs being used for the two race nets but the race nets were sometimes interlaced with each other with one frequency pair on standard Six Hundred KHz separation straddling one of the frequencies of the other nets repeater. We did not experience the kind of de-sensing or damage that you are warning me of. So what you are saying is at odds with most of the Net Control operations that I have seen at a large number of larger public service events. We have had Twenty Watt continuous output FM ATV repeaters co-located with a voice repeater with both on the Seventy Centimeter Band without the issues that you are reporting with a vertical separation of only ten feet between the two antennas.

Your SWR is going to be all messed up, because you have too many of the
same antenna's in the same proximity.

The antenna handbook says that two Wavelengths of separation from other metallic objects is sufficient to prevent de-tuning at VHF UHF frequencies. Are you saying that because the two antennas are co-resonant that the effect is that much worse.

The only good 6 / 2 / 70 cm antenna that I could recommend would be the
Diamond v2000. This is the only antenna that I am aware of that has a
decent amount of gain - if you want to call it that, along with being
semi resonant on all three bands.

What you are doing is back-feeding everything that you transmit back
into the receive of the front ends of all of the radios in your shack
when any one radio transmits. Unless it is in your budget to replace
all those radios on a semi annual schedule, you will eventually
experience that each of those transceivers will eventually become deaf.


Others have said that the DCI-146-444-DX-DB Diplexer and Dual Band Filter will provide sufficient isolation. Since you are saying that is not true would you mind explaining why?

I have seen filters promoted in QST that allows two operators on two
different bands to share a beam antenna with two transceivers, as long
as each transceiver stays on it's band it is ok.

But there is a hell of a difference between 20 meters - 14 MHz - CW and
40 meters Phone.

We have used such a filter set on our 20, 15, 10 Meter tribander for the last several field days with excellent results and no ill effects. Two of the transceivers used were my Yaesu FT-1000 and FT-857D. I have had no performance changes on either radio.

Even though 70 cm is not a harmonic of 2m, and even though there is a
heck of a disparity between 440 MHz and 146 MHZ there is always going to
be problems when dealing with FM, and Digital modes.

What kind of problems?

I have to take your wife's side on this one!

My Wife's objections are aesthetic. She doesn't want our home to look like a police precinct station when viewed from the street. So what exactly are you agreeing with her on.

Tell your club to go out and buy an acre of ground and put up a
transmitter and a tower and put their packet and their D-Star crap on
their tower, and then you can tune to their tower frequency if you so
choose. You are killing not only all of your transceivers by what you
are trying to do, but you are diminishing the range at which you
yourself can operate...

How am I accomplishing that exactly? I have already had several VHF UHF transceivers operating at the same time at my home without harm to any of the equipment. The only change I would be making here is that more than one transceiver would be operating on different bands on a multiband antenna using the DCI-146-444-DX-DB and/or other DCI Four Pole Band Pass Filters for isolation.

If you can hear other repeaters / more than 20 miles away though all of
that RF noise you have created, you will be lucky..

I've already done this with separate antennas at this location without any loss of range or intelligibility that I could discern. I have run the Baltimore Washington Skywarn net with the Radio Message Server Packet chirping away only twenty feet apart at the same height above ground level (AGL) without either one missing a beat. That includes a net that covered an activation for a tornado watch that actually produced three tornadoes and a very busy net. All of the repeats and fills were caused by over excited operators rather than by de-sensing.
It doesn't matter if the radios are all turned on or off, as long as
they are connected to the coax / antenna, they are still going to
experience front end overload.

--
Channel Jumper

I realize that I didn't say this previously but I do use grounding coaxial switches on my rigs and they are always in the shorted grounded condition when the radio that they are serving is turned off. I also have both Surge protectors on all my antenna coax entries and EMP protectors on all of the rigs. I don't see how enough energy to damage the rigs could get past the EMP Protectors.

--
Tom Horne W3TDH