Michael Black wrote:
[snip]
If they'd given thought to propagation, they'd
not have allocated 27MHz to a band intended for local use.

Even with nobody working skip, when the skip was in the band got cluttered.
You couldn't avoid propagation getting those distant signals to you, and
that did not help the intended use of the band.

Michael VE2BVW

Michael,

I don't feel it was a question of propagation: the FCC wanted to create
an inexpensive radio service that could be used by those who could not
afford the standard "two-way" radios that were available at the time.

The problem, in a nutshell, was that tube designs for VHF and UHF were
very expensive to manufacture, tune, and repair. I think the idea with
the class C & D Citizens' Bands was to make the rigs affordable _using
the vacuum tube designs of the day_, and that meant keeping the
frequencies low. Since ham operators had an assignment at 10 meters, and
the primary user of 11 meters (Medical Diathermy) wouldn't be affected,
it probably seemed like the ideal spot: the only commercial allocations
near it were in the 30-50 MHz range, but assigning a new service there
would have meant displacing existing FM licensees, including many local
government users, who were _also_ interested in keeping their radio
costs down.

Don't forget: the class A & B Citizens' Bands had been authorized for
many years, with dismal results: since class A & B CB radios used 460
MHz, users had to pay for the same Motorola or GE or Johnson sets that
telephone, utility, taxicab, and others with deep pockets were using.

Long story short: experience had shown that the cost of UHF units was
too high a barrier for farmers and other rural users, and IMHO, _that_
was the reason for choosing 27 MHz.

YMMV.

William

(Filter noise from my address for direct replies)

Since ham operators had an assignment at 10 meters, and
the primary user of 11 meters (Medical Diathermy) wouldn't be affected,
it probably seemed like the ideal spot: the only commercial allocations
near it were in the 30-50 MHz range, but assigning a new service there
would have meant displacing existing FM licensees, including many local
government users, who were _also_ interested in keeping their radio
costs down.

I remember medical diathermy in the 50's being an important cause of
TVI. They apparently were available for home use by patients. I never
needed one so I really have no idea what they were supposed to do.
To they still exist? Have they been replaced by a different
technology?

Jon W3JT

In article ,
Jon Teske wrote:
Since ham operators had an assignment at 10 meters, and
the primary user of 11 meters (Medical Diathermy) wouldn't be affected,
it probably seemed like the ideal spot: the only commercial allocations
near it were in the 30-50 MHz range, but assigning a new service there
would have meant displacing existing FM licensees, including many local
government users, who were _also_ interested in keeping their radio
costs down.

I remember medical diathermy in the 50's being an important cause of
TVI. They apparently were available for home use by patients. I never
needed one so I really have no idea what they were supposed to do.
To they still exist? Have they been replaced by a different
technology?

Medical diathermy sources come in two kinds: electrocautery systems used
for surgery, and deep tissue heating. The deep tissue heating systems were
the big offenders, since they were often left operating for long periods
of time.

The deep tissue heating systems have most been replaced with ultrasound,
although some RF-based deep tissue equipment still exists. Electrocautery
is still around.

Aside from the reduction in use of deep-tissue heating systems, the RF
characteristics have been improved a lot. Some early systems were just
relaxation oscillators, running more or less in an ISM band with lots of
impressed 60 Hz trash and lots of harmonics. That stuff has all been
cleaned up and modern medical devices are now clean enough that you
could add a key to them and use them on 10M.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Thanks for anwering my original question.

Jon W3JT

On 20 Apr 2007 11:04:36 -0400, (Scott Dorsey) wrote:

In article ,
Jon Teske wrote:
Since ham operators had an assignment at 10 meters, and
the primary user of 11 meters (Medical Diathermy) wouldn't be affected,
it probably seemed like the ideal spot: the only commercial allocations
near it were in the 30-50 MHz range, but assigning a new service there
would have meant displacing existing FM licensees, including many local
government users, who were _also_ interested in keeping their radio
costs down.

I remember medical diathermy in the 50's being an important cause of
TVI. They apparently were available for home use by patients. I never
needed one so I really have no idea what they were supposed to do.
To they still exist? Have they been replaced by a different
technology?

Medical diathermy sources come in two kinds: electrocautery systems used
for surgery, and deep tissue heating. The deep tissue heating systems were
the big offenders, since they were often left operating for long periods
of time.

The deep tissue heating systems have most been replaced with ultrasound,
although some RF-based deep tissue equipment still exists. Electrocautery
is still around.

Aside from the reduction in use of deep-tissue heating systems, the RF
characteristics have been improved a lot. Some early systems were just
relaxation oscillators, running more or less in an ISM band with lots of
impressed 60 Hz trash and lots of harmonics. That stuff has all been
cleaned up and modern medical devices are now clean enough that you
could add a key to them and use them on 10M.
--scott

William Warren ""w_warren_nonoise\"@comcast(William Warren).net" wrote:

Don't forget: the class A & B Citizens' Bands had been authorized for
many years, with dismal results: since class A & B CB radios used 460
MHz, users had to pay for the same Motorola or GE or Johnson sets that
telephone, utility, taxicab, and others with deep pockets were using.

Actually there were some cheap two-tube units out there, which used
a single device as a regenerative detector or as an oscillator, combined
with an audio amplifier tube. The performance was very poor, however.
"Vocaline" was the manufacturer that I remember.

Long story short: experience had shown that the cost of UHF units was
too high a barrier for farmers and other rural users, and IMHO, _that_
was the reason for choosing 27 MHz.

Yes.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."