My low freq antennas evolved along the same lines (no pun intended).
Ultimately, you just need to try it.
The "dipole" can be almost any length, and need not be resonant. Whatever
the lowest freq you want to work 80M, then make the total antenna length =>
1/2 wave and as high as possible. Try it on lower freqs, it might work
(altho not efficient). If you want to work DX (beyond, say 500 miles), then
consider a vertical, inverted-L, or something "non-dipole". But try
everything and anything...everyone seems to want easy answers and no
experimentation, and in reality, lots of things work (sort of) and may just
be fine. Don't believe everything you read on the internet or the ham
reflectors.
600 ohm line is easy to build and is somewhat less lossy than 450 ohm line.
Either is OK. A balanced tuner may work better than an L or Pi network (your
Dentron), but again, try it, it'll probably work OK. It may give you some
difficulty on 160 or lower freqs or full-limit power, but try it, what have
you got to lose?
The difference in lengths & heights of your antennas seems insignificant to
me. Try both, use whichever gives least RFI and best performance.
Experiment.
To bring balanced line into the shack, what I have is two runs of 1/2"
Heliax, about 6" apart, total maybe 6' length, drilled thru the joists. The
balanced line is crimped + soldered to the Heliax and once inside the shack
goes to an EFJ tuner. I don't use a balun (but you could try one). I don't
bond the shields of the Heliax together or to ground (but you could try
that). It works fine. I used to have a 260' doublet at ~ 50' but it was too
hard to keep in the air with our rain/ice/wind, so I shortened it to ~ 130',
now around 50' high and it loads fine on 80/40 but I have up on 160M.
Plenty of info available on the internet, if you want more details, contact
me.
Bottom line, just about anything will work, some better than others, read
the old handbooks, try stuff, and don't believe all the ham radio gospel
everybody is quoting today....the Old Timers made stuff work, and nobody
told them it wouldn't!
You mention RFI in the house...this suggests issues of grounding (electrical
and antenna), feedline/tuner issues, and proximity to your household
devices. Are you sure you need to change your antenna, versus improve your
present installation? Each situation is different, you need to make sure
you're using best practices and haven't overlooked something. Do you have an
Elmer or club in your area to play Devil's Advocate and help find things
you might have overlooked? Could be your antennas are fine but some other
tweaks might help?
Good luck and 73
dan
k0dan
-----Original Message-----
From: Jim Miller Waco Texas WB5OXQ
Sent: January 15, 2012 06:31 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Dipole fed with balanced line?
I must move my 40 and 75 dipoles due to rfi in the house that I cannot
resolve. I am thinking about 1 dipole fed with ladder line or window line.
I have 2 possable support points.
1 pair is 126' from end to end and 50' high at one end and 30' at the other.
Second pair 139' apart but 25' at one end and 30' at the other. The longer
but lower is farther from the house by about 10' and runs over level yard
with no obstructions. What is up with the 130' dipole fed with balanced
line? Is this better for a multiband antenna than using 1/2 wave at the
lowest desired freq? I have a MT-3000A Dentron tuner which is supposed to
work with ballanced line. The maximum power I run is 800 watts rms not pep.
Suggestions please. I have never used a dipole fed with balanced line.
Will the window line take 800 watts or should I get the open wire feeder? I
really only need this antenna to work on 75 and 40. I use my triband beam
for 20+. How do you bring this line into the shack? Obviously it cannot
touch anything metal along its run. Is the length of the feeder critical?
Should the wire be long as possable or 1/2 wave at the lowest desired
frequency? (3.9mhz)
wb5oxq
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