I had planned to sit on the sidelines and enjoy the spirited dialog, but
Jim brought up a point which needs to be chiseled in stone:
Communications vans are NOT the ideal environment for a repeater!
Case in point: A nearby Air Force Base has a mobile command post which
is, for all intents and purposes, a communications van. I invited the
officer in charge of this vehicle to bring it out to a radio club
meeting for a "show and tell" to which he readily agreed. It was a
wonderful experience for the members of my Amateur Radio Club to
witness, first hand, the superior technology that the communication
wizards had employed to create this masterpiece of emergency capability.
Our joy at observing this epitome of radio communications capability was
diminished when one of the hams asked what all of the antennas (UHF
mobile 3dB gain) lined up, 10 inches apart at the rear of the trailer,
were used for. The officer replied that each of the antennas was
connected to a separate Motorola Astro Digital Spectra radio, so that
multiple conversations could be carried on, using the Base's UHF trunked
radio system. When asked if two or more conversations had ever been
handled at one time, the answer was, "Uh, no, for some reason we can
only talk on one radio at a time." DUH, Hello!?
Rest assured, your local, state, or federal taxpayer dollars are being
spent for fiascos such as this, simply because these communications
vans/trailers/command posts are seldom being designed and engineered by
radio-savvy people, but by catalog browsers and bean-counters. Not only
is a 10-inch spacing between the antennas of two same-band transceivers
an invitation to disaster, not to mention potential damage to nearby
radios, but none of the comm van designers seemed to understand the
realities of desense and bandpass filtering.
Of course, it is a challenge to install bandpass filters on
frequency-agile radios, but if you want to operate independently in a
dense RF environment, you must design your system accordingly. The
primary channels should be on single-frequency radios, with extremely
tight bandpass filtering on all receive frequencies and, if necessary,
on all transmit frequencies.
When properly designed, a communications van/trailer/command post can
operate simultaneously on a multitude of frequencies, bands, and
emissions. Unfortunately, far too many such installations are doomed to
failure before a switch is thrown.
73, Eric Lemmon WB6FLY
"Jim B." wrote:
>
> Running that much power in a communications van *WILL, REPEAT, WILL*,
> cause interference to other radios in the van, and is TOTALLY
> UNECESSARY! 10-20 watts is PLENTY for a 'portable' repeater.
> And in most communcation van environments, there is not enough room for
> anything the size of a 2M or even a 220 duplexer. A 10 watt UHF repeater
> in a communcations van with a 3dB gain antenna on 30' to 60' of mast is
> quite adequate for anything you would need a repeater in a comm-van for.
> If you need more coverage then that, you need to deploy more then just a
> comm-van anyway.
> Of the active comm-vans I am familiar with, only one has a repeater on
> board, and it never gets used in a response, cause it usually causes
> more problems then it cures, to the extent that they are thinking of
> pulling it.
> --
> Jim Barbour
> WD8CHL
>
>
> Yahoo! Groups Links
>
>
>
>
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