On Sun, 7 Jul 2002 13:13:30 -0400 "Tom Rauch" <w8ji@contesting.com>
writes:
> When I lived in Rockdale county, despite having a uphill slope
> towards Europe and rocky ground, a 35 foot vertical almost always
> tied my 120 foot or so high dipole for long DX on 80. (W8JI)
>
N4KG: EVERYTHING seems to work better to Europe from GA
than from ALA :-( My HIGH Dipoles seem to play better
to Central EU than my GP but the GP is often 6-10 dB
better to LA / SM / OH.
> > N4KG: SO, you AGREE that a TOP LOADED Vertical is superior
> > to a BASE LOADED vertical over a limited radial field
> > typical for most city lot limited users?
>
> Depends. Like many things, it is a juggling act.
>
> As the loading coil height is increased in a basically uniform cross
> section radiator, coil loss for a given design generally increases
> while ground loss decreases. The change in efficiency depends on how
> poor the ground system is, how poor the coil is, and if the antenna
> has a top-hat of significant dimensions.
>
> There will be some point that optimizes the results.
>
> The larger the top hat, the less critical the coil becomes, so I
> would agree that top hat loading is a better choice, but
> unfortunately that also means the coil location is NOT important!
>
> In other words, I'd be really careful to look at things before just
> moving the coil up high. I would not hesitate to add a large
> capacitance hat at top, because that makes everything less critical.
N4KG: GOOD EXPLANATION. Thank you.
>
> > In other words, a short tower / tribander, shunt fed
> > (or even set up as an Elevated GP) would be a
> > MUCH better antenna than a Base Loaded (HF2V)
> > vertical on a small city lot. (N4KG)
>
> I don't know how you would feed a grounded tower as a groundplane,
> since by definition it is grounded and the ground path can have a
> large effect on efficiency. My choice would be to do something
> predictable, like install a good ground system and use traditional
> shunt feeding.
N4KG: Understand. My 'Reverse Fed Elevated GP' *plays*
very well to EU / JA / UA0 on 80M with another tower 70 ft
to the South which I assume acts as an inefficient
reflector based on modeling of 2L arrays and comparisons
with another vertical in the SE corner of my 4 acre lot
which plays better to SA / LP. (See JUNE 1994 QST
or ARRL Antenna Book...Note 54 ft of feedline coiled
up at the feedpoint...may provide some feedline isolation).
N4KG: I *believe / feel* that this 80M Elevated GP plays better than
a base loaded 32 ft vertical over a short (1/8 WL) radial field.
N4KG: I *believe* this is a good and reasonable option for those
with space limitations, especially since it eliminates the tower
interaction problem by using the tower as the radiator.
N4KG: Shunt feeding a 40 to 50 ft tower / beam against ground
radials is probably even better, assuming a reasonable
radial field can be installed.
W8JI: I am not recommending anything, other than people do the best
>they can and experiment. Some people, as unbelievable as it might seem,
> will do pretty well with a short vertical whether base loaded or
> not..even if they can not get in a perfect or near perfect ground.
N4KG: W4NS / K4CEF did amazingly well on 160M with a shunt
fed 100 ft R25 Tower Top Loaded by a TH7 and NO radials.
I can't explain it but he was 'competitive' in the pileups.
> My only point is to add technical accuracy to some points of this
> discussion, not to tell people what will work when there are dozens
> of unknowns.
>
> People generally can make better decisions with facts or
> understanding of HOW something interacts, rather than anecdotal
> evidence.
>
> > N4KG:What do you recommend to the guy who lives on a 60 by
> 120 ft lot?
>
> I only recommend they look at what is around them, try to the
> understand any resources they have, and decide what they want
> to try in order to get the results they might want.
>
> They only clear thing in any of this is to avoid magic antennas, and
> use systems with good engineering.
>
N4KG: AGREE. Thanks for the enlightenment on the topload / coil
trade-offs.
>
> 73, Tom W8JI
> W8JI@contesting.com
>
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