Very well stated. Our county just bought such a trailer, and they put a man
in charge of this unit that believes he knows everything about radio
communications, to the point that he argued with me for nearly an hour when
he told me that our Hustler G7 antenna at our local Ambulance Service was no
good becuase he checked it with a continuity checker and it showed a dead
short. (This was after a lightning storm fried the radio). As I tried to
explained to him that it would show a short as that is the design of the
antenna, he became so flustard that he ended the conversation with "That is
the Wrong Antenna for This Radio", VHF Radio and a High Band VHF Antenna, 50
Ohms, figure it out, but my Bird Watt Meter Was wrong, as well as my MFJ
Analyzer, yet the antenna receives and transmits just fine. Anyone can
build a communications van, but to make it work, that is a different story.
And as far as power goes, I started out with a Regency Repeater when I made
the decision to build a repeater for our county. The exciter is 3 watts out
and was being heard better than 15 miles away with no problems on a
handheld, and there was not even a duplexer on it yet. It did this
everyday, using seperate antennas. Now we run 160 watts through a duplexer,
and not many people noticed the difference. Better way to say it, the
better basketball players are generally taller. Not power, but height.
Mathew
>
> 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
> >
> >
> >
> >
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
>
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/Repeater-Builder/
<*> To unsubscribe from this group, send an email to:
[EMAIL PROTECTED]
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
