Request for Information -- 75 ohm impedance antenna to 50 ohmimpedance BNC input

Dave Plowman (News) wrote:

I wish to connect an antenna with 75 ohm impedance coaxial output to
the receiver.

Making the physical connection is not a problem with a suitable
converter, but an anybody suggest a supplier of a suitable transformer?

I'd say the losses of the transformer likely to outweigh any gains.


RF transformers are very good, they have come on a great deal in the
past few years.

Losses of 1dB are easily achieved at 100 MHz with transformers from
Mini-Circuits or Coilcraft.

In article ,
Ian Jackson wrote:
In message , "Dave Plowman (News)"
writes
In article ,
J G Miller wrote:
I wish to connect an antenna with 75 ohm impedance coaxial output to
the receiver.

Making the physical connection is not a problem with a suitable
converter, but an anybody suggest a supplier of a suitable transformer?

I'd say the losses of the transformer likely to outweigh any gains.


Exactly. In any case, it's highly unlikely that the receiver input IS a
very good 50 ohms (an/or that the aerial feed is a very good 75 ohms).
Just plug it in, and use it. Also, if the aerial feed happens to be
fitted with a 75 ohm BNC, that will mate OK with the receiver's 50 ohm
BNC.

but you may never get a good contact with a 50 ohm connector again (you may
not want to). With many makes the centre pin on the 75 ohm version is
thicker than on the 50 ohm one.

--
From KT24 - in "Leafy Surrey"

Using a RISC OS computer running v5.11

In message , m.t6
writes
Ashley Booth wrote:

I have tried several Web searches but have not found anything
appropriate, perhaps because I am not using the correct search
terms, so can anybody provide help with the following problem.

I have a radio receiver with a 50 ohm impedance BNC input.

I wish to connect an antenna with 75 ohm impedance coaxial output
to the receiver.

Making the physical connection is not a problem with a suitable
converter, but an anybody suggest a supplier of a suitable
transformer?


Try Mini-circuits.

http://www.minicircuits.com/products/imps_main.html


Oops. I've linked to Pads, but they also do RF transformers.

Failing that you can always look around for a old spectrum analyser
converter, but they usually have N type connectors rather than BNC.

Just connect it direct and see if it works to your satisfaction. IMHO
it's not worth matching the small impedance mismatch for a radio
receiver.

I use a 50/75 Ohm matching pad (Mini-circuits) on my spectrum analyser
but that has a 5.7dB loss. Not what you need on an antenna i/p!



I assumed that J.G Miller was already using the receiver and wanted to
give a impedance matched system a go. Sometimes it is better to have
a matched system with a couple of dB worth of loss.

Remember filters only work properly with the correct impedances on
the input and output.

If he has a circuit diagram, and he might be able to modify the
RF stages to suite his needs, but not everybody has the skill to
do that.

Completely OTT. Except for very specialized applications (none of which
I have ever yet come across), it isn't going to make a scrap of
difference. Just plug the aerial feed in, and use it.
--
Ian

In message , charles
writes
In article ,
Ian Jackson wrote:
In message , "Dave Plowman (News)"
writes
In article ,
J G Miller wrote:
I wish to connect an antenna with 75 ohm impedance coaxial output to
the receiver.

Making the physical connection is not a problem with a suitable
converter, but an anybody suggest a supplier of a suitable transformer?

I'd say the losses of the transformer likely to outweigh any gains.


Exactly. In any case, it's highly unlikely that the receiver input IS a
very good 50 ohms (an/or that the aerial feed is a very good 75 ohms).
Just plug it in, and use it. Also, if the aerial feed happens to be
fitted with a 75 ohm BNC, that will mate OK with the receiver's 50 ohm
BNC.

but you may never get a good contact with a 50 ohm connector again (you may
not want to). With many makes the centre pin on the 75 ohm version is
thicker than on the 50 ohm one.

With BNCs, I've never found that a 75 ohm male a 50 ohm female don't
mate 100% (and vice versa). The main 'metallic' difference is that the
75 ohm pin is usually tapered more (ie more pointed), and the 50 ohm is
more rounded. Electrically, the 50 ohm male also has more PTFE
dielectric between the pin and the outer leaves. But essentially, you
can slap them together with impunity. [But note: 50 and 75 ohm
N-connectors DO NOT mix.]
--
Ian

"Ian Jackson" wrote in message
...
In message , "Dave Plowman (News)"
writes
In article ,
J G Miller wrote:
I wish to connect an antenna with 75 ohm impedance coaxial output to
the receiver.

Making the physical connection is not a problem with a suitable
converter, but an anybody suggest a supplier of a suitable
transformer?

I'd say the losses of the transformer likely to outweigh any gains.


Exactly. In any case, it's highly unlikely that the receiver input IS
a very good 50 ohms (an/or that the aerial feed is a very good 75
ohms).

I'll second that.

--
Martin

Roger Mills wrote:
In an earlier contribution to this discussion,
J G Miller wrote:

I have tried several Web searches but have not found anything
appropriate, perhaps because I am not using the correct search terms,
so can anybody provide help with the following problem.

I have a radio receiver with a 50 ohm impedance BNC input.

I wish to connect an antenna with 75 ohm impedance coaxial output to
the receiver.

Making the physical connection is not a problem with a suitable
converter, but an anybody suggest a supplier of a suitable
transformer?

What you need is an appropriate *balun*.

The technical term for this answer is that it is complete and utter
b******s! A balun is used to match a BALanced device to an UNbalanced one.
The degree of mismatch on receive means it is not worth worrying about.
Connect it and it should work OK.

Peter Crosland

In article , Roger Mills
scribeth thus
In an earlier contribution to this discussion,
J G Miller wrote:

I have tried several Web searches but have not found anything
appropriate, perhaps because I am not using the correct search terms,
so can anybody provide help with the following problem.

I have a radio receiver with a 50 ohm impedance BNC input.

I wish to connect an antenna with 75 ohm impedance coaxial output to
the receiver.

Making the physical connection is not a problem with a suitable
converter, but an anybody suggest a supplier of a suitable
transformer?

What you need is an appropriate *balun*.


Nope theres on need for any balanced to unbalanced conversion to be
done. Just use it as others have described the input Z prolly won't be
exactly 50 ohms anyway...
--
Tony Sayer

"J G Miller" wrote in message
...

I have tried several Web searches but have not found anything appropriate,
perhaps because I am not using the correct search terms, so can anybody
provide help with the following problem.

I have a radio receiver with a 50 ohm impedance BNC input.

I wish to connect an antenna with 75 ohm impedance coaxial output to
the receiver.

Making the physical connection is not a problem with a suitable
converter, but an anybody suggest a supplier of a suitable transformer?

Just connect it without any matching device. It won't make any difference.
What with the losses in the matcher and the liklihood that the receiver
won't be exactly 50ohms I'd be very surpried if a matcher could improve
things at all.

Bill

On Tue, 19 Feb 2008 15:05:33 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
J G Miller wrote:
I wish to connect an antenna with 75 ohm impedance coaxial output to
the receiver.

Making the physical connection is not a problem with a suitable
converter, but an anybody suggest a supplier of a suitable transformer?

I'd say the losses of the transformer likely to outweigh any gains.

I believe DAB is suppose to be 50 Ohms. When I got my first DAB tuner
about 6 years ago I rigged up the aerial with 50 Ohm cable and it
worked fine. When I eventually changed it for 75 Ohm cable, there was
very little difference.

Marky P.

In article , Marky P wrote:
**J G Miller wrote:
I wish to connect an antenna with 75 ohm impedance coaxial output to
the receiver.

Making the physical connection is not a problem with a suitable
converter, but an anybody suggest a supplier of a suitable transformer?

I'd say the losses of the transformer likely to outweigh any gains.

I believe DAB is suppose to be 50 Ohms. *When I got my first DAB tuner
about 6 years ago I rigged up the aerial with 50 Ohm cable and it
worked fine. *When I eventually changed it for 75 Ohm cable, there was
very little difference.

It's a different frequency band as well, which makes a bit of a nonsense of
my DAB/FM hi-fi tuner using the same connector for both services, but I just
plugged in the Band II 3-element folded dipole I've been using on FM for
years, and it works fine on both systems. At least, the DAB signal strength
or quality indicator, or whatever it is, shows full scale on most
programmes, so presumably it's working as well as it can. It still doesn't
sound as good as FM though.

Rod.