Hello,

Should have asked this also in my previous post re 50 or 75 ohm coax, but
I guess it is probably better to start a different thread.
New at this, so please pardon these very basic type of questions.

For a receiving only application, 30 MHz on down:
Putting up a new Inverted-L receiving antenna.

Radio, Balun, Lightning Arrestor use the 259 type of plug which I guess is
nominally for 50 ohm systems.

I can go with 75 ohm BNC connectors for the main coax run, but am pretty
well stuck with the
259 type for the already purchased units I noted above.

Intuitively, I guess I would like (all) 75 ohm connectors if I am using 75
ohm coax.

But, for 30 MHz and lower listening, would I even notice any difference,
probably ?

Is any received signal lost, or reflected back, at these 75-50 ohm junctions
?
If meaningful, any way around the problem ?

Thanks,
Bob

ignore it, you will never hear the difference.

"Robert11" wrote in message
. ..
Hello,

Should have asked this also in my previous post re 50 or 75 ohm coax, but
I guess it is probably better to start a different thread.
New at this, so please pardon these very basic type of questions.

For a receiving only application, 30 MHz on down:
Putting up a new Inverted-L receiving antenna.

Radio, Balun, Lightning Arrestor use the 259 type of plug which I guess is
nominally for 50 ohm systems.

I can go with 75 ohm BNC connectors for the main coax run, but am pretty
well stuck with the
259 type for the already purchased units I noted above.

Intuitively, I guess I would like (all) 75 ohm connectors if I am using 75
ohm coax.

But, for 30 MHz and lower listening, would I even notice any difference,
probably ?

Is any received signal lost, or reflected back, at these 75-50 ohm
junctions ?
If meaningful, any way around the problem ?

Thanks,
Bob

On Sat, 25 Feb 2006 15:32:06 -0500, "Robert11"
wrote:

Hello,

Should have asked this also in my previous post re 50 or 75 ohm coax, but
I guess it is probably better to start a different thread.
New at this, so please pardon these very basic type of questions.

For a receiving only application, 30 MHz on down:
Putting up a new Inverted-L receiving antenna.

Radio, Balun, Lightning Arrestor use the 259 type of plug which I guess is
nominally for 50 ohm systems.

Firstly:

Some types of connectors are designed to create a minimal
characteristic impedance discontinuity, examples are N type and BNC.
PL-259s are not so designed.

Secondly, at HF, the impedance discontinuity of PL-259 or 50 ohm BNC
in a nominally 75 ohm line for your intended use is insignificant.

Beware of having 50 and 75 ohm versions of BNC and like around,
incorrect pairing risk damage to the female connector. Most 75 ohms
applications still use 50 ohm BNC connectors, even though 75 ohm
versions are available.

Owen


I can go with 75 ohm BNC connectors for the main coax run, but am pretty
well stuck with the
259 type for the already purchased units I noted above.

Intuitively, I guess I would like (all) 75 ohm connectors if I am using 75
ohm coax.


See above re connector damage.

But, for 30 MHz and lower listening, would I even notice any difference,
probably ?

Is any received signal lost, or reflected back, at these 75-50 ohm junctions
?

Yes, we can readily calculate the effect of introducing a
discontinuity, the effect is so small in your application that you are
unlikely to be able to measure it, much less see the effect in
receiver signal strength.

Owen
--

Robert11 wrote:
Is any received signal lost, or reflected back, at these 75-50 ohm junctions

Simply forget about that worry. PL-259s are not 50 ohms.
That's why they are not used at VHF/UHF.
--
73, Cecil http://www.qsl.net/w5dxp

Robert11 wrote:
Hello,

Should have asked this also in my previous post re 50 or 75 ohm coax, but
I guess it is probably better to start a different thread.
New at this, so please pardon these very basic type of questions.

For a receiving only application, 30 MHz on down:
Putting up a new Inverted-L receiving antenna.

Radio, Balun, Lightning Arrestor use the 259 type of plug which I guess is
nominally for 50 ohm systems.

I can go with 75 ohm BNC connectors for the main coax run, but am pretty
well stuck with the
259 type for the already purchased units I noted above.

Intuitively, I guess I would like (all) 75 ohm connectors if I am using 75
ohm coax.

But, for 30 MHz and lower listening, would I even notice any difference,
probably ?

No. You wouldn't even if you were transmitting.

Is any received signal lost, or reflected back, at these 75-50 ohm junctions
?

No significant amount. The length of the discontinuity is extremely
short compared to a wavelength, and it's not a terribly bad
discontinuity. You'd be hard pressed to measure the difference at HF,
even with very good equipment. (Of course, you could see it with a high
speed TDR, but that's not an HF measurement.)

Roy Lewallen, W7EL