On Nov 3, 2005, at 1:48 PM, Jim Lux wrote:
> At 12:24 PM 11/3/2005, Bob Nielsen wrote:
>
>> On Nov 3, 2005, at 12:02 PM, Jim Lux wrote:
>>
>>>
>>>
>>> A rotary joint shouldn't be a big deal for HF. We use a variety
>>> of N
>>> connector rotary joints on the antenna range at JPL, and they are
>>> fairly
>>> wide band. I would imagine you could find one surplus.
>>>
>>
>> I used rotary joints for many years when I was working on antenna
>> ranges, but don't recall ever seeing any coaxial models with which I
>> would feel comfortable running amateur legal limit power levels.
>
> The one I have here, a 2 channel unit, is apparently rated at more
> than a
> kW. I don't know what the limiting component is (is it HV
> breakdown or
> thermal dissipation).
I have mainly used single channel (N or SMA connector) types and
recall that the N units would handle >1 kW peak, but only ~100 W or
less avg, so it is probably thermal dissipation (this was several
years ago). I recall that they used brushes and/or mercury for the
rotating contacts. I have used high power units with quarter-wave
chokes as a non-contacting joint, but of course that wouldn't be
practical at HF.
I just did a Google search and found a model from MI Technologies
which was rated for 1 kW average at 1 GHz, so it is possible (no
price was given).
> I suspect that they aren't cheap if purchased brand new.
> OTOH, if you wanted to make one, for HF, I suspect that you could
> get "good
> enough" if you had access to a competent machinist. It's going to be
> hundreds of dollars, regardless.
> It might be interesting to scrounge up one of the rotating sign
> slipring
> units and test it RF. They can certainly handle the power. Sort
> of like
> repurposing AC power relays for antenna switch boxes. Not everyone
> needs a
> Transco or Dowkey switch good to 18 GHz at 1.01:1 VSWR.
True, quite often what we do professionally clouds our thinking when
it comes to amateur projects, when a cheaper approach would be
adequate. Now that I am retired, I look at things somewhat
differently :^)
About 40 years ago I made some rotary joints for the NASA ATS-1
satellite using TNC connectors, but they only had to handle 10 watts
at VHF and rotate ~135 degrees one time to deploy the antenna
elements once the satellite was in orbit (by the time we got through
testing them they weren't all that cheap, however). That part of the
satellite operated for many years past its design life.
73,
Bob, N7XY
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