On 08/10/2011 11:45 AM, Tim Shoppa wrote:
On Jul 24, 3:18 pm, Kenneth wrote:
For the most part I will be using coax fed antennas.

There is an idea I've been thinking about and I wonder if it will work.
There is a myth that high SWR on coax will produce a high line loss.
Most of the loss involved with having a high SWR is in the coupling to
the receiver / transmitter, not so much in the transmission line itself,
at least not at HF. So long as a high voltage is no present on the
transmission line (arc over) or a very high current (resistive loss),
and the dielectric material does not introduce loss at the frequency
involved common 1/2" dia coax should work fine as an unbalanced feed
line even at a high swr ratio.

Coax loss at high SWR is a myth? It's been tabulated in every handbook
for the past century.

Common Ferrite core balums do not usually work well at impedances above
50/75 ohms input. Using them to convert an unbalanced transmatch to a
balanced one AT THE OUTPUT is usually not a good idea.

Often it's the choice made, though.

Baluns that deal with less than optimal impedances are available but
they are no magic bullet. Substantial engineering effort and material
must go into them to deal with the stresses at operating away from
their natural sweet spot (as you point out 50 ohm ballpark.)

(Insulate the
transmatch from ground and put the balum at the input works better).
A "choke" balum made by winding 10 turns or so of coax in a loop about a
foot or so in diameter has lower loss than a ferrite core balum, no core
heating.

So my idea was to run a length of coax from the shack through the attic
crawl space and up to the roof (though a vent turbine). On the roof
would be a coax choke balum connected to 450 ohm twinlead "open wire"
line. This would connect to a "multiband" dipole. There is no way I
can run the open wire line from the shack up to the roof, this stuff
must be run 'in the clear' anyway. Coax being an unbalanced line is not
affected by it's surroundings.

The coax to 450 ohm twinlead via choke "balun"... wow, it sounds like
you are getting the worst of all possible worlds here. I agree that
transformer-type baluns are stressed at high impedances but
purposefully putting a 9:1 mismatch on the end of your coax is just
asking for a world of hurt with anything except the most shortest coax
runs (a few feet.)

You might want to back up a few steps and consider if you can locate a
coax-to-twinlead tuner, almost certainly not broadband, on the wall or
roof of the shack or house. That way you have a nice controlled 50 ohm
coax run inside, and it's all twinlead outside. If the shack is far
from the tuner, today there are remotely operated tuners just for this
purpose. In the most traditional set ups mechanical remote operation
(chains, pulleys, etc.) was sometimes used (look in any 30's/40's/50's
handbook.) (When I was a kid my uncle who was a ham, used a scheme
involving bicycle cranks and extra long chains for rotating his beam!)

Len Cebik's webpages are an excellent resource on ladder line, all
band doublets, and link-coupled tuners. I can't emphasize enough how
excellently this can work out. Don't paint yourself into a corner.

Tim N3QE

If you run coax up to the roof, how about using some 70 ohm CATV coax?
I have some here I got for free, 3/4 in diameter. Oftentimes cable TV
companies have shorter 'reel ends' they are willing to give away. The
stuff is 'semi-rigid' but you may be able to get it to feed up that
conduit anyway. Worth a try. (BTW I do *not* mean RG-6 junk here, I mean
the larger diameter semi-rigid types only)

On the roof, if you must go the 'balun' route, a 4:1 or more likely 9:1
unun may be OK. Obviously, a balanced tuner accessible for adjustments
would be best.

CATV 'hardline' is not near as lossy as RG-8 or RG-213 types up around
10 meters. So try some- and don't worry that it's not 50 ohm impedance,
just use it as if it were.

I built the SPC type tuner myself, nearly 20 years ago. It's not always
the answer, but oftentimes it worked pretty good, allowing me to
extend the usable BW of some narrow antennas and still keep the amps
happy.

73, David K3KY