On 11/3/2010 1:25 AM, Jim Brown wrote:
>> Add 0.25-0.50 dB for any connectors/adaptors if used above 50Mhz
> This is another one of those old wives' tales that has no basis in fact
I can vouch for that Jim., I've mentioned a number of times on here
about the number of connectors I have in the system.
Once I leave the tuner, I have the connector on the tuner (1), a cable
to the patch panel (2 more) bulk head connector on the patch panel (1),
LMR 500 to the 6-pack at the base of the tower (2) connectors going
through the 6 pack (2) not counting relays inside. Jumper to bulkhead
connector on hoffman box (2). Bulkhead connector (1) jumper to Grounded
bulkhead connector at bottom of tower (2), bulk head connector (1) 100'
of coax to the top of the tower (2) bulkhead connector (1),
pigtail/rotator loop from bulkhead connector to antenna (2) connector on
antenna (1)
Now these antennas can be fed from either the house or shop through the
6-pack for SO2R although it works equally well for 2O2R
That's a total of 20 connectors between the back of the antenna tuner
and the antenna terminals for each run. I have one run for 160, one for
the 6-meter yagi, one for the 40 meter yagi, and one each for 144 and
440. That's a total of 5 runs ALTHOUGH there is the side mounted
144/440 antenna at 50' and two remote antenna switches for the sloping
dipoles.
On the air tests between a single run with all those connections or a
nearly continuous run to the tribander that was up there using antenna
switches to switch back and fourth showed no discernable difference on
receive or transmit.
IF there was some difference even a quarter db with each of 20
connectors would give 5 db, or for all practical purposes a full S
Unit. At a half db each I'd be looking at 10 db difference or basically
the same as turning my amp on or off on transmit which would certainly
be noticeable. It wasn't!
73,
Roger (K8RI)
> -- they are the big lie that is believed simply because it is so often
> repeated. The loss in any decent RF connector is a tiny fraction of a
> dB. W8JI has a rather simple way of helping us think through the
> silliness of your statement. 0.5dB is a power ratio of 0.8913. If
> you're running 1.5kW through that connector, a loss of 0.5dB is 163
> watts. Think about the heat from a 163 watt light bulb inside that
> connector. Or with a high duty cycle typical of contesting, 50 watts
> average power.
>
> Such large losses only occur with junk connectors, like those with the
> tiny springs for a center conductor. Now, I DID experience that kind of
> heating with that sort of junk connector, at at that power level -- it
> was VERY HOT after less than an hour of contesting. When I replaced it
> with an Amphenol, it ran cool as a cucumber.
>
> This spring, VE1RAC sent me documentation of some very good experimental
> work he had done showing that the loss in decent connectors at VHF is
> FAR lower than that -- a few HUNDREDTHS of a dB, as I recall.
>
> 73, Jim Brown K9YC
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