On 7/1/2015 12:26 PM, Jeff Liebermann wrote:
On Tue, 30 Jun 2015 15:13:55 -0400, Jerry Stuckle
wrote:

Yes, it's most effective to match the feedline to the antenna at the
antenna connection. But it's also important to match the transmitter to
the feedline.

This latter piece is often ignored because people will use a feedline
who's characteristic impedance matches the transmitter already (i.e. 50
ohm line for a 50 ohm transmitter).

However, there are exceptions. For instance, if you're feeding a 75 ohm
antenna (i.e. a dipole) with 75 ohm coax, a 1:1 balun at the antenna
will provide a good match (ideally, 1:1). But there will be a 1.5:1
mismatch to a 50 ohm transmitter. In this case it would be better to
have the matching network at the transmitter.

We may have had this discussion before. Matching a 75 ohm load to a
50 ohm source might be academically interesting, but the actual loss
is almost negligible. for a VSWR of 1.5, the return loss is 14dB and
the load mismatch attenuation is 0.177dB. That's about what I would
expect to lose in two coax connector pairs.

You could also feed the antenna with 50 ohm feedline and place the
matching network at the antenna. The effect would still be a 1:1 SWR,
but the lower impedance of the coax would create higher i^2R losses; not
important if you're talking a short line, but a longer one would lower
output at the antenna.

True, but for roughly equivalent sizes of coax cables, the 75 ohm
cable has less loss and the equivalent 50 ohm cable. If you want to
handle high power, use 50 ohms. If you want low loss, use 75 ohms:
http://www.belden.com/blog/broadcastav/50-Ohms-The-Forgotten-Impedance.cfm
Note that these are for air dielectric cables.

Things are not so neat if we consider the dielectric. See the bottom
paragraph and graphs:
http://www.microwaves101.com/encyclopedias/why-fifty-ohms
Dielectric Dielectric const Minimum loss impedance
solid PTFE 2.2 50 ohms
foam PTFE 1.43 60
air 1.0 75
RG-6/u CATV 75 ohm foam coax still has slightly less loss than the
equivalent 50 ohm cable, but not as much as I've previously claimed.

This is cute:
http://cablesondemandblog.com/wordpress1/2014/03/06/whats-the-difference-between-50-ohm-and-75-ohm-coaxial-cable/
"A good rule of thumb is that if the device being connected
via coaxial cable is a receiver of some kind, 75 Ohm Coax is ideal."



Jeff, do you always miss the forest for the trees? That was an EXAMPLE.
The same would be true if you were feeding a 300 ohm yagi with 300 ohm
twinlead and a transmitter with a 10 ohm output impedance.

And BTW - when calculating, you forgot about the transmitters which cut
back power to protect the finals. Many will do so even with a 1.5:1 SWR.

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