Mike,
Look at Wikipedia for "mismatch loss" to see how there can be loss
without heat in an RF system.
There are 2 kinds of loss: dissipative (heats up) and mismatch (produces
no heat).
If a connector was lossless but was of an impedance different than the
line there would be mismatch loss with no heating.
Let's use a PL-259/SO-239 as an example. A mated UHF connector looks
like about 0.9" of 30 ohm transmission line. Make the connector
lossless for example and in a 50 ohm system a mated PL-259/SO-239 would
produce a 1.43:1 SWR at 450MHz. This will result in a mismatch loss of
0.138dB. At 1000W, 0.138dB of dissipative loss would be 3.2W which
would make the connector slightly warm. Add a few other UHF mated
connectors and the SWR will generally rise (however they may actually
cancel but leave a section of transmission line with an elevated SWR
which will cause additional dissipative loss).
So the correct answer is: Given low loss connectors of the correct
impedance produces very little if any additional loss but can easily
produce mismatch loss if the wrong connectors are used (or if they are
terminated to cable incorrectly). This can be a big deal at UHF.
73,
Larry, W0QE
On 4/30/2012 2:55 PM, Mike Waters wrote:
>> On 4/29/2012 11:01 PM, Charles Harpole wrote:
>>
>>> I know ever coax joint, every coax switch, and so on costs rcv db.
>>>
>> ... This is one of those often repeated myths with little basis in fact.
>> A properly done coax connection is essentially lossless at HF. ...
>>
> Exactly. It wouldn't take hardly any loss at all for a connector, switch,
> or whatever to get warm fairly quickly when power was transferred through
> it. If it did have significant loss, you would burn your fingers on the
> connector at well below the 1500 watt level.
>
> For example, a connector with 0.1 dB loss would dissipate ~35 watts of heat
> at 1500 watts. Do you have any idea just how hot a resistor dissipating
> that much power would get? I guarantee you that you would quickly yank back
> your hand in pain as soon as you touched it.
>
> Nobody's coax connectors, properly installed, get hot. And even at only 100
> watts, a connector with .1 dB loss would still get warm!
>
> 73, Mike
> www.w0btu.com
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