Attached is a summary to my inquiry a week or so ago about top loaded
verticals, especially with regard to what form of top loading - loading
coils versus umbrella top hats versus wires, and whether drooping top hat/
umbrella wires was a good idea. I hope reflector members will find it useful.
Thank you VERY VERY MUCH to all who provided food for thought.
Around two thirds of correspondents suggested the best way to go to was to
use a simple Marconi T antenna, i.e. two loading wires of equal length
going in opposite directions, on top of the vertical radiator. George
Guerin K8GG pointed out that these two wires were each approximately 70% of
the length of a single top loading wire, i.e the horizontal part of an
inverted-L antenna.
For about a 53' vertical section, this would mean I would need about 53'
per side to form a Marconi T antenna.
Bill K0HA pointed out that changing my antenna from an inverted L to a T
antenna would minimise horizontal radiation, but provide reasonable
efficiency. He adds that a loading coil that is near the top of a 50 - 55'
vertical section has to be VERY large (both in inductance and size) to
provide resonance on 160m. Bill also said that he would expect better
performance with the T antenna than a 50 - 55' high folded radiator using
an omega matching arrangement.
Chris G3SVL summarises from research he has done from the ON4UN book and
the W7XC article on top loaded verticals (QST March 1990) and produces a
list of five golden rules.
1. Coils in the antenna are always bad news (due to losses and problems in
getting a high-Q)
2. Top loading is always better than any other position for a coil*
3. If you use a coil, watch the dissipation - it can easily cook.
4. Top loading wires are ALWAYS better than a coil plus wire/capacity hat
5. Sloping wires reduce feed impedance and effective height (but are
better than a lossy coil).
With regard to whether drooping the top loading wires/top hat was a good
idea, Larry W7IUV pointed out that both computer modelling and scale live
modelling he had carried out that drooping top hat wires more than about 30
degrees from the horizontal was " a waste of time", ie, there was lots of
cancellation of radiation from the vertical section of the T antenna.
Several of the correspondents had successfully used two wire (K7CA) and
four wire top hats (G4VGO/EI7IU) which use drooping top loading wires.
However, Al K7CA made the excellent suggestion of minimising the
droop/slope of wires by using the longest possible lengths of rope attached
to the ends of loading wires, if there were not convenient high trees/masts
from which to attach them.
As regards matching the low impedance I would get with a Marconi T or L
antenna with a vertical section of around 50 - 55' (ie around 15 ohms if
the ground losses were really low) there were two suggestions. Whitey K1VV
and Charles N5UL suggested the way to go was to use a Unun (W2FMI) as these
were efficient and convenient (and can be multi-tapped in my experience).
Stan W7AWA who uses a T antenna with a 50' vertical folds the vertical
section of his antenna (by spacing two 50' long vertical wires about 1'
apart), which he says gives him a pretty good match to 50 ohm coax.
Other solutions to my desire for a vertically polarised antenna with a 55'
vertical section were generally along the lines of a short vertical
radiator with a top loading hat (LZ2CJ) or a top hat and a coil (G3SVL,
K0LW). Wally LZ2CJ passed on an interesting 160/80/75m design, based on a
14m vertical and a four-wire (each 3.1m long) top hat, with the wires
interconnected by a skirt wire. This design requires a sophisticated
relay-controlled matching unit.
* Larry W7IUV has found that field strength measurements and on-the-air
performance indicated to him that the best place for the a loading coil is
actually at the centre or just above the centre of a vertical. Larry says:
"any higher and the additional inductance required causes the coil losses
to skyrocket."
On a final note, owing to my problem with actually putting up a reasonable
T antenna here (antenna supports/earth mat in the wrong place!) I have had
a big re-think.
Earl K6SE recently suggested to me a very simple way to shunt feed my
three-section tower which supports the 160m inverted vee, using slant feed.
This means putting down a whole new earth mat, but would give me a
vertical antenna that is 80' high and top-loaded with a tribander yagi on a
19' boom - a much better vertical prospect than my badly placed 55'-high
tree supports can provide. While I shall have some fun with some of the
short vertical top-loading ideas provided, putting an earth mat under the
tower and slant feeding the latter is probably going to be the long term goal.
Vy 73,
Steve, VK6VZ
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