mprather(at)spro.net
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 Posted: Sun Feb 04, 2007 9:20 pm Post subject: Sharing ship's COMM antenna with the hand-held |
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One more comment... If you make a T in a transmission line and each leg
is of the same characteristic impedance there will be reflections
generated at the T. A device transmitting from one leg of the T will see
a section of the characteristic impedance line and then at the T the
impedance will appear to be half the characteristic impedance. The (any)
change in impedance in the transmission line will cause at least some
reflection. At a T each edge coming out of the transmitter will be
reflected at half the negative amplitude of the outbound signal. Needless
to say, this is likely to cause hi SWR and low transmitted power.
Regards,
Matt-
[quote]
<john(at)ballofshame.com>
Yes, of course you're right Dave. I was just thinking out loud, I guess.
-John
Dave N6030X wrote:
>
> <N6030X(at)DaveMorris.com>
>
> John, I don't think it's as simple as that.
>
> If you put a 50 ohm resistor at the end of a piece of coax you are
> transmitting into, the resistor becomes a "dummy load", and will heat
> up to the extent that it is rated near the power level of the
> transmitter. You cannot then simply add a T connector to another
> device and assume that the dummy load will be invisible to the system.
>
> If you connect a handheld radio with a 2W output into the receiver
> input of a panel mounted radio, you'd better hope that receiver's
> input section is capable of handling that level of power. I'm not
> sure any of them are.
>
> Dave Morris
>
> At 12:19 PM 2/4/2007, you wrote:
>>
>> <john(at)ballofshame.com>
>>
>> If you short a 50 ohm resistor across the end of a 50 ohm coax, you
>> will effectively remove that "leg" of the cable from the circuit so
>> far as back reflection is concerned. This is called terminating.
>> Basically, the 50ohm resistor acts the same as an infinitely long
>> coax, if that makes sense?
>>
>> So if radio manufacturers where clever, they would setup their
>> antenna inputs to short across a 50ohm load when the unit is off,
>> effectively terminating their end of the cable. On the panel, you
>> use a T with a 50 ohm terminator in the panel side. If you need to
>> use a handheld, you can turn off the panel mount and replace the 50
>> ohm terminator with your handheld input. The downside is that you
>> will loose range.
>>
>> Don't take any of this as a suggestion. It was just something that
>> flashed into my brain this morning when I thought about how to make
>> every compatible. I think this would work but you'd sacrifice
>> performance, so it's probably not viable.
>>
>> -John
>>
>> Doug Windhorn wrote:
>>>
>>> <N1DeltaWhiskey(at)comcast.net>
>>>
>>> John,
>>>
>>> Not sure what you are getting at here, but simply measuring the in
>>> "resistance" of the antenna input will, I think, tell you little.
>>> Or, maybe I am missing what you are suggesting.
>>>
>>> Did a quick look at "characteristic impedance" on Wikipedia. When
>>> one talks about transmission cables (and matching connections),
>>> impedance is the "apparent resistance" to AC signals that the line
>>> is rated in, not simple resistance. I believe you are talking about
>>> measuring the impedance, not solely the resistance, of the input.
>>>
>>> I could surmise some thing about this, but probably make myself look
>>> a little (maybe a lot) unknowledgeable, so will leave further
>>> clarification to those with a better understanding of the subject.
>>> But a good starting place for discussion is to have an understanding
>>> of the relevant terms
>>>
>>> Regards,
>>>
>>> Doug Windhorn.
>>>
>>>
>>>
>>>
>>> ---
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