Matching Coax Impedance: To Receiver or To Antenna ?
Robert11
Have a Scantenna antenna (receive only) in attic.
It came with 75 ohm RG 6 coax, so presumably the output has a 75 ohm
impedance.
How this varies with freq. I have no idea.
Have a new scanner that says to use 50 ohm coax.
From old posts, the consensus seems to be that it doesn't matter if you use
50 or 75 ohm coax for
a run that I have of about 50 feet.
Do you folks agree with this ?
From a somewhat more rigorous and theoretical view, should the coax, even it
hardly matters, be matched to the antenna output impedance, or the input
impedance of the scanner ? Why ? (again, receive only)
Any thoughts on this would be most appreciated.
Thanks,
Bob
dave
Telamon
"Robert11" <rgs...@notme.com> wrote:
Chances are I will be the only person to answer you that it matters.
Everyone else is of the opinion that it doesn't. You will lose received
signal power with the mismatch and the argument always revolves around
how much the loss is important to your reception.
Since you have the coax use it and if the performance is satisfactory
then I would not worry about it. If the performance is not good enough
then go buy the right impedance coax and for scanners at higher
frequencies the most important factor is loss per foot. You want the
lowest loss per foot in the frequency range you want to receive on.
For short wave the loss per foot is not as significant being in the 3 to
30MHz range. Coax loss goes up per foot at higher frequencies.
--
Telamon
Ventura, California
Telamon
dave <not...@nowhere.com> wrote:
That is a simplistic view.
--
Telamon
Ventura, California
The Traveller
Although you will theoretically see losses, the front-end gain of your
receiver is so high that it will compensate for this mismatch. The end
result will be unnoticeable unless the desired signal is at the noise
level (i.e. where the additional 1.3 dB would have an impact). As
noted, the losses will be more negligible on MF/HF versus VHF/UHF.
RG6 does offer some advantages over say RG58/U or RG8/8x cable in RX
on MF/HF frequencies: Good RG6 has a much better shield which will be
more efficient at blocking-off the pickup of unwanted interference.
Equivalent "50 Ohm" cables would be similar to the LMR-400, etc.
______________________
The Traveller
Carlsbad, California
RHF
- - Robert11 wrote:
- - Hello,
- -
- - Have a Scantenna antenna (receive only) in attic.
- - It came with 75 ohm RG 6 coax, so presumably the output
- - has a 75 ohm impedance.
- - How this varies with freq. I have no idea.
Neither Do I. {You are not Alone}
- - Have a new scanner that says to use 50 ohm coax.
Enjoy You New Scanner.
- - From old posts, the consensus seems to be that it
- - doesn't matter if you use 50 or 75 ohm coax for a run
- - that I have of about 50 feet.
- -
- - Do you folks agree with this ?
I Do. {See the Longer Answer Below}
- - From a somewhat more rigorous and theoretical view,
- - should the coax, even it hardly matters, be matched to
- - the antenna output impedance, or the input impedance
- - of the scanner ? Why ? (again, receive only)
It would be 'nice' to have a Signal Path where all the Parts
were the same Impedance : Antenna, Coax Cable and Radio.
-But- Practically Speaking Not Necessary when you are
simply talking 75 Ohm to 50 Ohm Components used for
Receive "Only" Scanner-Radio Listening.
- - Any thoughts on this would be most appreciated.
- -
- - Thanks,
- - Bob
Dave -wrote-
- The mismatch is insignificant.
- About 1.3 dB of loss or something.
Robert 11,
What Dave says is the Practical Applied Truth : When it comes
to these 75 Ohm to 50 Ohm Mismatches between Antennas,
Coax Cables, Radios, etc for the Average Scanner Radio Listener.
To correct this Mismatch although insignificant; would require
the Insertion of some 'Device' either between the 75 Ohm
Antenna and the 50 Ohm Coax Cable feed-in-line.
This Device would have to be 'connected' between the 75
Ohm Antenna and the 50 Ohm Coax Cable feed-in-line.
Each of the Connections would have a Insertion 'Loss";
plus the Device itself may have some "Loss".
DEVICE = Matching Transformer / Matching Network / etc.
The Total of these "Losses" may be nearly equal to
the original Loss from the 75 Ohm to 50 Ohm 'direct
connection' Mismatch; or could even exceed it.
So the Practical Answer is to accept the small 1.3 dB
Loss due to the Mismatch and have fewer Connection
in the Signal Path from your Antenna to the Radio.
Why Fewer Connections ?
Over time the biggest problem with most Antenna
installations is the Connections in the Signal Path
from the Antenna to the Radio. Two more Connections
may cause more problems then living with the small
Signal Loss due to the Mismatch between the 75 Ohm
Antenna and 75 Ohm Coax Cable and the 50 Ohm
Input of the Radio.
Worry less about these small Mismatches and Losses;
and Learn more about making good Secure Antenna
and Coax Cable 'connections' that are Weather Proof
and will hold-up over years of service.
sorry for the miss-match-of-words :o)
hope this helps - iane ~ RHF {pomkia}
.
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.
Unrevealed Source
makes a huge difference to match the long wire to the coax. RF Systems has
a magnetic longwire balun (MLB) that works very well.
I wouldn't worry about the 75/50 mismatch between the coax and the receiver.
It's the mismatch between the antenna and the coax that you have to address.
Jeff
"Robert11" <rgs...@notme.com> wrote in message
news:9tKdnQrRTs-Un1ba...@comcast.com...
Art Harris
> Have a Scantenna antenna (receive only) in attic.
> It came with 75 ohm RG 6 coax, so presumably the output has a 75 ohm
> impedance.
>
Get an adhesive label, write "50 Ohms" on it, and affix it to the coax
where the scanner can see it.
Seriously, RG-6 is good low loss cable. It will outperform a lossy
cable such as RG-58 (or even RG-8) which happen to be 50 ohms.
The impedance of your antenna will vary considerably with frequency.
But even if it had a constant 50-ohm impedance across its range, the
use of 75-ohm cable will only result in a 1.5:1 VSWR. For a 50 foot
run of RG-6 cable, the loss due to that VSWR will be a fraction of a
dB. Don't worry about it. You're way ahead of the game due to the
inherent low loss of RG-6.
Art Harris N2AH
dave
> I don't know about scanner frequencies, but at shortwave frequencies it
> makes a huge difference to match the long wire to the coax. RF Systems has
> a magnetic longwire balun (MLB) that works very well.
>
http://users.belgacom.net/hamradio/schemas/Magnetic%20Longwire%20Balun_MLB.htm
The RFS is way overpriced. If you're going to spend that kind of
money get something not made by communists, e.g. a Palomar MLB-1.
Unrevealed Source
haven't taken the RF Systems balun apart (like you said, they are costly)
but for some reason they work MUCH better. Worth every penny.
"dave" <not...@nowhere.com> wrote in message
news:13so171...@corp.supernews.com...
dave
RHF
wrote:
-
- I've actually tried some of the cheaper ones and also
- built my own. I haven't taken the RF Systems balun
- apart (like you said, they are costly) but for some
- reason they work MUCH better. Worth every penny.
UrS,
"Secrets of the Universe Revealed" a Look-in-Side
the RF Systems Magnetic Longwire Balun (MLB)
http://www.remeeus.eu/english/hamradio/mlb.htm
X-Rays and A-Lot-More . . .
~ RHF
.
> "dave" <noth...@nowhere.com> wrote in message
>
> news:13so171...@corp.supernews.com...
>
>
>
> > Unrevealed Source wrote:
> >> I don't know about scanner frequencies, but at shortwave frequencies it
> >> makes a huge difference to match the long wire to the coax. RF Systems
> >> has a magnetic longwire balun (MLB) that works very well.
>
> > So does a home made one for $10.
>
> >http://users.belgacom.net/hamradio/schemas/Magnetic%20Longwire%20Balu...
>
> > The RFS is way overpriced. If you're going to spend that kind of money
> > get something not made by communists, e.g. a Palomar MLB-1.- Hide quoted text -
>
> - Show quoted text -