I'm renting a basement suite and have an Icom 706mkII & MFJ-949E antenna tuner, and I need a better HF antenna arrangement than I already have (without going to a tower, hi). I would like to run a balanced feedline from the tuner to the feedpoint but the feedline has to go from my shack to where the feedpoint is, and that involves crossing a cement path --- in other words, because the feedline has to lie on top of cement it can't really be anything other than coax. I've put tons of pictures and explanations and diagrams on my webspace at:
"http://members.shaw.ca/cyberhun/ham.htm"
--- please take a look and if you have any ideas let me know. I'm somewhat at a loss as to where to go with it now. I'm thinking the only way to significantly improve on the current arrangement is to run a feedline up from my shack, to the south-west corner of the roof. This would eliminate the need for coax (across the path) and then run radiating elements from the feedpoint to the south-west corner of the yard, and from the feedpoint to the south-east corner of the roof, maybe?
73 from canada
VA7FAB

In article ,
Thomas anonymous wrote:

I'm renting a basement suite and have an Icom 706mkII & MFJ-949E antenna
tuner, and I need a better HF antenna arrangement than I already have
(without going to a tower, hi). I would like to run a balanced
feedline from the tuner to the feedpoint but the feedline has to go
from my shack to where the feedpoint is, and that involves crossing a
cement path --- in other words, because the feedline has to lie on top
of cement it can't really be anything other than coax.

You could make up a section of shielded balanced line, to get the feed
across the cement without having it couple messily into the ground.

A shielded balanced line can be made from two equal-length runs of
coax cable. RG-6 would work; aluminum-jacketed cable-TV hardline
might be even better from an electrical standpoint although its
rigidity might be inconvenient. Connect the shields/braids of the two
sections of coax together at each end, use the center conductors
to carry the signal, and fasten the two coaxes together in some
convenient fashion (an occasional nylon wire-tie would do, I believe).

The impedance of such a line will be twice that of the individual
pieces of coax - i.e. 150 ohms if you use RG-6. No need to worry
about matching it to the rest of the balanced feedline. You can use
300 or 450 or 600-ohm (nominal) balanced line where convenient, to
keep the losses as low as practical, and this 150-ohm shielded
balanced pair where necessary. The losses should be acceptable, and
will certainly be less than if you plumbed the whole distance with 50-
or 75-ohm coax.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

In article ,
(Dave Platt) wrote:

In article ,
Thomas anonymous wrote:

I'm renting a basement suite and have an Icom 706mkII & MFJ-949E antenna
tuner, and I need a better HF antenna arrangement than I already have
(without going to a tower, hi). I would like to run a balanced
feedline from the tuner to the feedpoint but the feedline has to go
from my shack to where the feedpoint is, and that involves crossing a
cement path --- in other words, because the feedline has to lie on top
of cement it can't really be anything other than coax.

You could make up a section of shielded balanced line, to get the feed
across the cement without having it couple messily into the ground.

A shielded balanced line can be made from two equal-length runs of
coax cable. RG-6 would work; aluminum-jacketed cable-TV hardline
might be even better from an electrical standpoint although its
rigidity might be inconvenient. Connect the shields/braids of the two
sections of coax together at each end, use the center conductors
to carry the signal, and fasten the two coaxes together in some
convenient fashion (an occasional nylon wire-tie would do, I believe).

The impedance of such a line will be twice that of the individual
pieces of coax - i.e. 150 ohms if you use RG-6. No need to worry
about matching it to the rest of the balanced feedline. You can use
300 or 450 or 600-ohm (nominal) balanced line where convenient, to
keep the losses as low as practical, and this 150-ohm shielded
balanced pair where necessary. The losses should be acceptable, and
will certainly be less than if you plumbed the whole distance with 50-
or 75-ohm coax.

perhaps just making a wood board or somthing to fill a few inches and
cut to allow the window to close on it, w/foam too and drill a hole in
the wood to pass the wire and have it centered enought away on all
sides from metal then it dosn't disturbe or damage the building/window
etc

Tunnel under the sidewalk and run coax through a pvc pipe to your antenna?
Sidewalk tunnelling is relatively easy with a "wand" and hose.

Hmmmmm, never thought of that ! Something to think about, for sure ...

Hey Dave, about the 'shielded, balanced line' idea --- you know how the whole idea behind using twinlead (or any balanced feedline, I guess) instead of coax is because twinlead (or any balanced line, i suppose) is supposed to much less lossy --- I'm wondering if I were to make a balanced line that is also shielded, like you suggested, would this not provide the best of both worlds? It would have the convenience of shielded coax (able to be run in places that twinlead couldn't) AND perhaps also be less lossy than coax? Or would it? How lossy would such a balanced, shielded feedline be? Do you know, offhand, Dave?
I'm not well versed in RF circuitry. All I can really do is look at it in terms of it being a simple resistor, in which case, I suppose having 2 parallel feedlines would have half the ohmic losses (I think), but there's clearly a whole lot more to it than just this.


73 from grid CN89od

Thomas,

I'd suggest a remote tuner. You can have twinlead directly to the
tuner, and then your entire run of coax sees a nice flat 50 ohms.

A nice automatic tuner would be ideal; you don't have to worry about
controls. The price is not exactly right, since you've already got a
tuner...

If you've got a well stocked junk box (op amps, power transistors,
maybe a few multiturn pots) you could try my approach. I put a small
gear motor and sensing potentiometer on each shaft of my MFJ tuner,
stripped the spring detent balls out of the inductor switch, added a
servo circuit (very simple, it's an op-amp and a couple of transistors)
and can use the tuner more or less as if it were in the shack.

See http://www.n3ox.net/projects/servo for more information if you're
interested. Your MFJ-949 would lend itself nicely to this approach.
It can be done on the cheap, maybe.

A problem arises if you don't have 10-turn (or at least 360 degree)
pots... I've been thinking about this in that they're sort of the weak
point of my tuner project, as they're something like $15 per new. I
work in a physics lab and every few months an obsolete homebrew
instrument with a load of them hits the dumpster, so I'm set.

I think remote tuner of some description would be superior from a loss
standpoint to any feedline tricks you could do.

Now, the balanced, shielded line on the concrete is a good idea if
you're set on running twinlead all the way back to the shack. This is
quicker and easier than than building a remote control for your tuner,
and cheaper than buying an autotuner, so you might try it first and see
if you're happy with the results.

In my apartment, I originally ran about 20 feet of 75 ohm coax between
the antenna feedpoint and the tuner. I made some contacts, but saw a
DRAMATIC difference when I put the tuner at the antenna feedpoint
instead. Fewer RF feedback problems too.

Twinlead is better but twinlead isn't magic. Wide spaced open wire
line with an insulator every couple of feet only is super low loss even
with high SWR. 300 ohm twinlead, AFAIK, isn't.

73,
Dan
N3OX
www.n3ox.net