[Elecraft] Folded Dipole Antennas

Fri Dec 5 11:39:03 2003

The antenna tuner resonates the "antenna system" comprising the antenna =
and the feedline. Now the impedance seen at the input to the open line =
can be way too low or high for a balun to work properly. How then does =
the tuner do it? A combination of line loss (even with open line there =
can be >1dB loss due to extremely high VSWR)and balun loss and =
everything appears to be fine but much of the tx power is heating the =
system. For my twin lead fed inverted vee I ran simulations to determine =
an antenna length and feedline length that would keep the magnitude of =
the impedance (at the tuner) below 1k ohms. For the 40-10 meter antenna =
I use a 15 meter inverted vee fed with 24 feet of 300 ohm line. It is =
fed differentially on 20-10 and has good modeled efficiency. On 40-20 it =
is fed as a vertical against ground and two 23 foot radials. On all =
bands it has a good radiation pattern. It also tunes as a vertical on 80 =
and while the efficiency is not high it is enough for casual use.

   Dave

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Don Wilhelm
Sent: Thursday, December 04, 2003 6:32 PM
To: Bob Tellefsen; Edmund T. Wright; [email protected]
Subject: Re: [Elecraft] Folded Dipole Antennas

Bob is correct, a resonant length is not a requirement for an antenna.  =
As
long as you can feed it effectively with small losses, any length will
radiate effectively.  Open wire or ladder line feeders will keep losses =
down
to a minimum.  More information in my antenna article on my webpage
www.qsl.net/w3fpr for those who are interested.

If you are interested in effective dipole lengths not resonant on any =
band,
see the information on L B Cebik's website www.cebik.com .  He advocates =
an
88 ft. lenght for 80 through 20 and a 44 ft. length for 40 thru 10 =
meters -
his choice of lengths is because of radiation patterns produced - i.e. =
the
88 ft version has only a single lobe up through 20 meters and breaks =
into
multiple lobes at higher frequencies - likewise, the 44 ft. length has a
single lobe pattern through 10 meters.

The real story is that one can use an antenna of most any length and =
have it
radiate efficiently, as long as you can keep the losses to a minimum and
bring the impedance at the transmitter end of the feedline to something =
that
can be matched to 50 ohms.  Beware, the length of the feedline makes a =
big
difference in feeding non-resonant antennas.  If you are experimenting, =
make
notes of the length and type of feedline as well as the length of the
antenna.

73,
Don W3FPR

----- Original Message -----

> Actually, Edmund, when you have the K2 you can just
> use a plain dipole fed with 450 ohm line and let
> the KAT2 match it for you.  I use a 60 ft long dipole
> here, not resonant on any band, and work 80-10m with it.
> 73, Bob N6WG
>
>

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