Sorry for getting in on this late. Various forms of wife-inspired
busy-ness.
The real kicker here isn't the pattern, it's LOSS. It is helpful when
mentally eyeballing an installation like this to see it in *wavelengths*
rather than linear feet.
The distance that a dielectric material can be from the radiating source
and cause a given degree of loss increases with increasing wavelength. It
all has to do with how much total energy it takes to raise the molecular
energy of a dielectric molecule to the next level.
While the energy level at a distance is weaker, if the push is for a long
enough time, the weaker energy of the radiation multiplied by time can
accomplish the raise to the molecular next level. Since a 160 meter signal
is pushing in one direction much longer during the half cycle than the
microwave signal in your microwave oven, you don't have to be in the two
foot box to get dielectric effects. The math is really nasty, and it's not
a linear reduction formula but linear is close enough to *characterize* the
damage in loss by "not high enough" or wrong polarization.
A pair of vastly shortened MA160V radiators produces a heavily loaded 72'
dipole at 55 feet in your example. That's approximately a 1/8 wave dipole
at a height of 1/10 wavelength.
To visualize, let's convert that to the ten meter band, where the
wavelength is 33 feet, to get the real view of what's going on. That ten
meter antenna would be a four foot loaded dipole supported at a height of
forty inches above ground. You tell me how well that ten meter antenna is
going to get out, and whether you think it will heat up the ground.
Prior comments about the cloud burner pattern are correct, but even worse
it's a severely weakened cloud-burner antenna. And for 160m there's more.
Famously, W8JI put up a 160 dipole that was up a little over a half wave,
supported from the tops of two appropriately spaced 300 foot towers. His
analysis, after using that and his excellent vertical antenna farm in
innumerable A/B tests, was that the dipole RARELY EQUALED the verticals in
signal strength.
There are reasons why 160 meters goes vertically polarized, and the
efficiency of the counterpoise is the first, second, and third place
consideration in what to do and where to put it.
With the right counterpoise (another very very long discussion) the MA160V
can be made to work halfway decently. The hint here is that shortened
antennas have a very low real radiation resistance, meaning higher current
for the same *radiated* power, and higher local RF field intensity because
it's not spread out as much as a quarter wavelength radiator.
Thimble summary of long long counterpoise discussion?
Do not screw around with any aspect of gimpy, underfunded, half-a**ed,
afterthought, miscellaneous, irregular, undersized counterpoise on 160m.
Counterpoise is the 1000 pound gorilla in a room otherwise occupied by mice
and small monkeys. Guess who's crushing chairs and hammering the piano into
toothpicks. Treat the Gorilla right or the Gorilla will make you pay.
Doesn't matter whether you're a really nice guy, or gave to the PBA and the
bell-ringers, and coach a little league team, and you're the best dad there
ever was. Screw around with the counterpoise and the Gorilla will make you
pay. Every time.
Don't believe this? Free country for sure. And it's fun to experiment. Just
be sure and come back and let us know your 160 operation in one or the
other of the 160 contests with that antenna so we can look up your RBN
numbers and put a dB cost to the experiment.
The tipping point in the decision is whether you truly enjoy trying
something different in antennas just to see for yourself what happens (go
for it!), or you are trying to do something in restricted circumstances and
your REAL goal is getting quickly to actual radiated power adequate for
your choice of QSO's.
In any event, good luck and have fun.
73, Guy K2AV
On Tue, May 24, 2016 at 1:02 PM, GALE STEWARD via Topband <
topband@contesting.com> wrote:
> Totally agree. It would have to A LOT higher to obtain anything but a
> cloud warming radiation pattern.
> Stew K3ND
>
> From: Hardy Landskov <n7rt@cox.net>
> To: w5jmw@towerfarm.net; topband@contesting.com
> Sent: Tuesday, May 24, 2016 11:58 AM
> Subject: Re: Topband: ma160v
>
> Don't waste your time & $$. It's a cloud warmer which is good SS contests &
> local QSO parties.
> 73 N7RT
>
> -----Original Message-----
> From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of
> w5jmw@towerfarm.net
> Sent: Monday, May 23, 2016 9:40 PM
> To: topband@contesting.com
> Subject: Topband: ma160v
>
> Hi all.I currently have 1 cushcraft ma160v,as you all know this is a
> cushcraft 160 vertical.I just recently aqquired a second.Now,has
> anyonetried
> a rotatable dipole using 2 of these.The idea intrigues me.I could mount
> this
> on a 55 ft tower.The only other antennas are a m2 6 mtr
> 2 klm's 2 mtr and a 432 m2.So there should not be any interaction.What does
> the group think? any ideas? a waste of time? your thoughts..73 john w5jmw
>
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>
>
>
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|