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

Dot wrote:

Cecil Moore wrote:
Simply forget about that worry. PL-259s are not 50 ohms.
That's why they are not used at VHF/UHF.

Looked at the back of a 2 meter rig lately?

I should have said they are not used at VHF/UHF by rational
human beings. di-di-di-dit---di-dit.
--
73, Cecil http://www.qsl.net/w5dxp

On Sat, 25 Feb 2006 16:47:28 -0800, Roy Lewallen
wrote:

Owen Duffy wrote:

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.

I keep reading this, and it puzzles me. I have a number of RG-59 patch

Roy,

I have clear recollection of 75 ohm BNCs (meaning a BNC connector
desiged for 75 ohm through impedance) that were incompatible in days
when I worked in communications facilities that used 75 ohms for IF
(70MHz) and baseband, and 50 ohms for RF (GHz).

However, I note on Amphenol's site:

"Two distinct types of 75 ? BNC's are available, and both mate with
each other and with 50 ? BNC's. Type 1 is designated 75 ? BNC-T1 and
provides constant 75 ? performance with low VSWR DC – 4 GHz. Type 2 is
designated 75 ? BNC-T2 and is usable with low reflection DC - 1 GHz.
For applications above 1 GHz, Type 1 is recommended. "

Though it is clear that Amphenol and probably some other manufacturers
produce compatible connectors, I am mindfull of earlier experience,
though I concede that it may have been the result of non standard 75
ohm BNCs.

Owen
--

On Sat, 25 Feb 2006 16:47:28 -0800, Roy Lewallen
wrote:


I don't know about type N connectors -- I don't think I've ever seen a
75 ohm version.

The 75 ohm version N-type uses a reduced centre pin and I believe they
are incompatible.

Owen
--

"Owen Duffy" wrote in message
...
On Sat, 25 Feb 2006 16:47:28 -0800, Roy Lewallen
wrote:


I don't know about type N connectors -- I don't think I've ever seen a
75 ohm version.

The 75 ohm version N-type uses a reduced centre pin and I believe they
are incompatible.

Owen
--
Problem I had was getting the center conductor of 50 Ohm coax to go into the
hole at the back of the 75 Ohm pin. I think they mate OK, because the skinny
part of the pin has the samo OD.

Tam/WB2TT

"Roy Lewallen" wrote in message
...
Owen Duffy wrote:

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.

I keep reading this, and it puzzles me. I have a number of RG-59 patch
cables which have 75 ohm BNC connectors fitted (and others with 50 ohm
connectors). As far as I can tell with a magnified inspection, the center
conductor of the 75 ohm male BNC is the same diameter as it is in a
standard 50 ohm BNC. The only apparent difference is that the dielectric
around the inside of the outer mating area is much thinner in the 75 ohm
connector, and the diameter of the 50 ohm center pin steps up as it enters
the solid dielectric at the bottom while the 75 ohm center pin doesn't.

Since the pin of the male connector is wholly inserted into the socket, its
diameter should not matter. The OD of the *female* center conductor should
be different. Also, the OD of the back end of the male connector (the part
that does not insert) should be different. There were dimensions available
at the Tyco web site.

Tam/WB2TT