Long Wire fed without a Tuner

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Kurt Sweeny:
I will be visiting a friend with lots of space. I always wanted to put up a long wire. Can an adequate match be made with a balun without a tuner (assume monoband operation?) I have read that the input impedance is around 450 but this must be variable. I heard conflicting views on how to cut the antenna including avoiding multiple of the half wave (?) Without a tuner I assume it would be trial and error to get to the optimum length;  I would have to be able to separate the impedance mismatch from the non resonant length mismatch.



The location for the feedpoint will be near a tin roof;can the tin roof be used as passive element or as a pseudo ground plane (curious and maybe ridiculous.)

Bill Savage:
At resonance, a 3/4 or 5/4 wave antenna will have a relatively low impedance but it will still be somewhat above 50 Ohms. If your radio has an internal antenna tuner it will probably be able to match it. You WILL need two or more quarter wave radials (the more the better) to act as the "other half" of your antenna.

Cut the long wire to the length L = (246 x N)/f, where L is in feet, f is in MHz, and N is an odd number; 3, 5, 7, etc. This will result in a slightly long length which you can trim until you achieve the desired resonance point.

And yes, this will be a single-band antenna.

Cecil A. Moore:
Quote from: KD6LZV on May 25, 2010, 09:10:12 AM

I always wanted to put up a long wire. Can an adequate match be made with a balun without a tuner (assume monoband operation?) I have read that the input impedance is around 450 but this must be variable.

Is this a receive-only antenna? It is possible for a long-wire to exhibit a resistive 450 ohm feedpoint impedance at a single frequency but it requires a counterpoise for transmitting because that impedance is still relatively low.

Please describe what you have in mind in more detail including more of "what you have read" and where you read it.

Maybe what you are talking about is a terminated long wire, i.e. a resistor to a good ground to reduce the reflections at the end of the long wire?
--
73, Cecil, w5dxp.com

Kurt Sweeny:
Thanks both for your comments.
Obviously I need to do some basic studying and would appreciate a reference.



Do the radials conconnect to the shield of the coax? if so, does the conductor completing the circuit require tuning?


I would like an  RX / TX antenna and read in a forum of a 9:1 balun being used to match the longwire to 50 ohms. (50 *9= 450) . A terminated long wire sounds like what I have in mind; I imagined an end fed long wire cut for resonance through some trial and error but didn't know where to start getting it to match to 50 ohms.

73

KD6LZV

Dale Hunt:
There are some sorts of terminated long wire antennas that give around a 450 ohm
impedance, but they aren't necessarily very efficient.

The input impedance of a random wire varies over a wide range depending on the
length in wavelengths.  It wasn't unknown for folks to transmit into such an antenna
with an old tube rig like a Johnson, some of which had output stages that could match
from 40 to 600 ohms.  But a solid state rig that shuts down when it sees 2 : 1 isn't
going to be very happy.

As K3AN posted, an antenna that is an odd multiple of a quarter wavelength will have a
relative low input impedance that comes pretty close to matching coax.  I did this
with a quarter wave wire on 75m with my first SSB rig, and while that doesn't exactly
constitute a "long wire antenna", it did get me on the band.  Something like a 5/4 wave
wire fed against the tin roof with an electrical quarter wavelength of 75 ohm coax
would probably work.  Longer multiples might match better with a 4 : 1 transformer
at the feedpoint, but it should be an UN-UN rather than a BAL-UN, because both
the antenna and the feedline are unbalanced.

If you consider a long center-fed doublet to be a "long wire", then using similar lengths
(perhaps 3/4, 5/4 or 7/4 wavelengths) on either side of the feedpoint might make an antenna
that matches well to a 4 : 1 balun in the center, as the input impedance increases as
the wire is made longer.

I have had good results feeding a loop antenna on multiple bands with a 4 : 1 balun and
using it on harmonics of the design frequency.  For example, a loop for 80m can be used
on 40, 20, 15 and 10m with reasonable SWR (as long as it is cut for the bottom of the
band to start with, in order to get the harmonics to fall properly.)  That isn't generally
considered a "long wire" antenna, but it is an antenna that uses lots of wire and space.

The next question is what you mean when you say "without a tuner".  Do you mean
without using a commercial box with knobs sold as an antenna tuner, or do you mean
any without any sort of impedance matching at all?  There is a huge range between the
two interpretations:  you can use sections of feedline to match inconvenient impedances
in various ways, and a simple L-network antenna tuner can be home-made from
hamfest parts that will match most end-fed long wires.  All it needs is a coil and a
variable capacitor (which could be made from a coil of fence wire and some empty
beer bottles in a wash tub, respectively, if you didn't have anything else handy.)  For
long center-fed doublets it isn't difficult to find lengths of the antenna and balanced
feedlines that give a good match to 50 ohms.  I've also made my own "balanced
tuner" using a quarter wavelength of open wire line stretched across the wall of my
garage (or between two trees outside) and matching impedances by sliding the
feedline connections along it until I get a good match.

There are various other antennas, like vee beams and rhombics, that use two (or more)
long wires that could be built.  The feedpoint impedance might be adjusted to around
450 ohms, but is often higher (and the wires probably would need to be tuned to length
for minimum SWR.)  Depending on the design, a length of parallel conductor line probably
could be arranged to match the antenna, either as a quarter wave matching section or
as a shorted stub.


So while a random end-fed wire fed against the metal roof probably will not be a good
match to your rig, there are many different options that might make a useful antenna
for you.

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