----- Original Message -----
From: "Tom W8JI" <w8ji@w8ji.com>
To: "Guy Olinger K2AV" <olinger@bellsouth.net>
Cc: <topband@contesting.com>
Sent: Tuesday, July 24, 2012 9:12 AM
Subject: Re: Topband: Beverage antennas
> Hi Guy,
>
> Good topic.
>
>> The losses as a reversible beverage would far exceed those used as a
>> balanced feedline, because of the balance partly cancelling fields in the
>> dielectric between them.
>
>
> Slow wave structures are more common in microwave. Anything that
> increases
> capacitance or inductance per unit foot will slow wave propagation. The
> formulas are 1/f*sqrt LC for wavelength, and 1/sqrt LC for phase velocity.
>
> I built short Beverages with multiple ferrite sleeves, because a thick
> dielectric was impossible. There are limitations for how slow we can make
> the wave that change with antenna length, after which the antenna
> reverses.
>
> This is why most slinky Beverage "theory" put out was nonsense, because
> the
> real action isn't packing a wave of wire in a small area....but rather
> slowing the velocity factor a correct amount. I *think* the limit for a
> half
> wave structure is a Vp of about .5 before the system reverses and starts
> firing backwards, but it has been years since I looked at slow wave
> structures for 160.
I dont know about your "most" comment but a Slinky Beverage works very well
as long as its kept under the direction reversing length.
Its easy to calculate the electrical wavelength and then the added loss and
wave slowing of the steel to come up with a workable version. I havent gone
beyond 5 of the full size Slinkys stretched over 175' which worked very well
for me for years. They require regular maintenance to retard rusting or the
brass versions can be used.
The Slinky is just one way to reduce the physical length, there is no
mystery involved.
Carl
KM1H
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|