OK, it's my turn -
On the long wire/vertical antenna topic:
For field day, we often use vertical antennas for 80 and 40 meters.
These are full size verticals, made from aluminum tower sections plus
tubing "stingers" to get the right length. There is no reason that
these same ideas cannot be used for the higher bands as well.
Each vertical antenna is surrounded by a batch of 16 to 24 radials, made
from plastic jacketed multistrand wire about 16 gauge (exact gauge
unimportant), and cut to some random size between 1/8 and 3/8
wavelength, laid on the ground. We use nails or weights on the far
ends to keep the wires more or less flat on the ground. We then put an
antenna coupler box right at the base of the antenna, and use the MFJ
antenna analyzer to adjust the system for resonance at the desired
operating frequency. BTW - if the MFJ box is connected directly to the
antenna, it usually shows about 35-40 Ohms impedance at resonance -
exactly what a 1/4 wave vertical should read! All of the radial wires
are connected to a common ground stake at the antenna base, of course.
The ground stake need not be very long - we use it just to provide a
common mechanical tie point.
If you are using an end fed wire (frequently mis-named a long wire), the
principles are exactly the same. Any end fed antenna, if operated
against earth ground, needs a fairly good ground system to be
effective. Antennas with one radial (or two, or three or four) may tune
up easily and show good SWR, but then so does a dummy load. An
interesting aside - to be a true "long wire", an antenna must be at
least one wavelength long, preferably several wavelengths.
The field day antennas are series fed (the base is insulated from
ground) via the antenna coupler. For KW levels, we use a Drake MN-2000
or similar. For QRP Levels, any of the small tuners should do the job
equally well. The proof, as they say, is in the doing. These
antennas really kick butt and take names. Remember, on Field Day, we
here in Alaska are fighting almost 24 hours of Summer daylight, and we
still manage a respectable number of contacts on both 80 and 40.
You don't have to build your antenna out of tower sections -- hanging
wire from a tree or other support will work just as well. But, there
really is a difference between one (or two) radials and 16 to 24 -
there are some charts in the antenna books that detail how much "return
on investment" you get from differing numbers and lengths of radials.
The way I read these charts, 16 to 24 random length radials represents
the best compromise between performance and effort/cost. Yes, when
trying to fit all this into a pack, it can be challenging, but small
gauge wire can be wrapped around a stick or other "media" (discarded
fishing leader spools are ideal), and kept in a zip-lock bag. And you
don't necessarily need to pack an antenna coupler - the one in the K2
(or whatever you have) will handle the task just fine.
Using a Balun might work, and it might not. In this situation, I would
opt for not using the Balun and just go for a direct connection. If
the antenna feed point is some distance away, use coax cable, and be
sure to connect the distant end of the coax to ground. Unfortunately,
this makes for an antenna that works well only on odd-numbered 1/4
wavelengths. A better approach for portable operations would probably
be direct feed (the antenna attached directly at the rig) - which gives
the possibility of multiband operation with an antenna tuner, such as
the one built into the K2 or other rig, assuming you have that option
installed. However, for semi-permanent installations, there is no
reason you cannot use the old "multiple element" trick connecting two
(or more) 1/4 wave elements for different bands to the same feed point.
If you do this, try to keep the "far" ends separate from one another by
at least a few inches, if possible. Antennas using traps are also a
good possibility. Traps can be made very small for low power operation,
and a cleverly designed antenna for portable use can use some of all of
the traps to operate on some of all of the bands for which it was built.
Good hunting!
73
- Jim. KL7CC
Sandy W5TVW wrote:
Have used vertically polarized antennas extensively "in the field" with added
radials/counterpoise wires. Usually laid on the ground.
Primarily QRP operations, where a small change can be detected readily!
I've found TWO radials the best compromise. Big improvement over one!
Adding more give slight changes for the better, but nothing of a large
difference.
I'd say four would be the maximum needed. More is always better as regards
radilas, but after 4 the returns increasingly diminish.
73,
Sandy W5TVW
----- Original Message -----
From: "John Rader" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Cc: "Elecraft Discussion List" <elecraft@mailman.qth.net>
Sent: Monday, August 02, 2004 10:03 AM
Subject: Re: [Elecraft] Long wire balun question
| Don,
| I currently have the antenna attached to a good earth ground and will add
| radials for each band per your suggestion. I understand that radials can
| significantly improve the efficiency of monopole antennas. Should I try to
| add more then one radial per band?
| Thanks,
| John K5XTX
|
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