[Elecraft] Matching End-fed Halfwave with K2/KAT2?

[Don Wilhelm]

Daniel,

Although an end fed halfwave antenna is easy to deploy, it presents a very
high impedance at the end.  The KAT2 has a difficult time matching a high
impedance load.

Your transformers may work (I can't really comment), but a simple parallel
tuned circuit (wound on a pill bottle and tuned with a capacitor) will
certainly work a lot easier.  You can do a 'rough match' with the tuned
circuit (fixed high voltage capacitor) and let the KAT2 refine the matching.
You would need a parallel tuned circuit for each band.  The coax attaches to
a link coil wound of a couple turns wound at the 'cold' end of the parallel
tuned circuit - the link has about 5% of the number of turns as the tuned
winding), or can be fed at a tap on the coil.  Experiment with your antenna
analyzer.

Another alternative would be to add a quarterwavelength of parallel feedline
to the end of the halfwave radiator - this is the classic Zepp antenna.  You
would need an electrical quarter wave cut for each band you will operate.
The quarter wave line transforms the high impedance at the end of the
halfwave down to a low impedance.

Of course, and even easier alternative is to cut the halfwave antenna in
half and feed it with the parallel feedline in the center - in that case it
should work on all bands with a parallel feedline sized to reach from the
antenna to the operating position.

I would believe that a W3EDP antenna - 85 ft. radiator with a 17 ft.
counterpoise wire would be just as easy to deploy as an end-fed dipole and
the KAT2 will handle it on all bands without the bother of the parallel
tuned circuit.  You may want to consider that as an alternative.

73,
Don W3FPR


> -----Original Message-----
>
> I'm considering using an end-fed halfwave for Field Day, and I
> have an idea how
> I might implement it, but need some technical assistance from those in the
> "know".
>
> What I'd like to do is pick up a few toroid cores (one to break,
> one to lose,
> and another to use) to build a simple transformer with a couple
> turns attached
> to a short (2-3 ft) pigtail of coax going back to the K2/KAT2.
> I'm hoping I can
> experimentally determine how many secondary turns to use for the
> halfwave side
> of things. Will the KAT2 be sufficient to handle the 'fine tuning' after I
> handle the major impedance transformation through the toroid? or
> will I need to
> get my hands on an air variable to do fine tuning on the antenna
> end of the
> pigtail?
>
> Any ideas on how to best implement this would be greatly appreciated -
> especially since I'll need to know which core(s) to buy this week
> so I can test
> this out before Field Day.
>
>
> If there's a cheaper way to do this using simple plastic pill
> bottles and air
> wound cores - I can do that too ... but I also like to wind toroids ;-)
>
> I'll probably end up with 1 or 2 antennas ... a 40/20/15 (40m
> halfwave), and
> maybe an 80/40/20 (80m halfwave). I may try to orient these crosswise to
> provide E-W coverage on 80/40/20 and N-S coverage on 40/20/15 -
> knowing that
> both of these antennas will start producing interesting lobes on
> the shorter
> wavelengths.
>
> Thanks for any help this list generates...
>
> 73,
> Daniel AA0NI
> Oklahoma City
> http://www.aa0ni.org
>
>