On Aug 31, 2:55 pm, Danny Richardson wrote:
On Fri, 31 Aug 2007 08:13:56 -0700, Tim Shoppa





wrote:
I am postulating a full-height quarter-wave 40M vertical that is
switchable (relays halfway up?) to a 20M vertical dipole.

Seeing as how my two favorite bands are 40M and 20M, seems to make
sense to me, but googling to see if anyone else has done this sort of
thing in the past doesn't give me a lot of hits. Is this workable? I
am not yet ready to go (budgetwise or real-estate-wise) to a real 20M
beam and I'd like to increase my oomph for DXing on 40M (right now I
have a wire dipole in the trees and am having a blast).

Looks to me I could build an extremely lightweight small 5M high tower
with a 5M metal mast on top. (I know, real conductor diameter will
mess those numbers up a little bit). And a relay between the mast and
the tower would either put them in series for feed as a quarter-wave
40M vertical, or separate the two and let me feed halfway up as a 20M
vertical dipole.

How would the ground radials used on 40M potentially mess up the use
as a 20M vertical dipole?

One idea for a lightweight 40M full-height tower/mast vertical with
insulated-from-ground mounting is shown athttp://svc.cc/antena.html

Take a look at this.

http://k6mhe.com/files/DualBandVert.pdf

73,
Danny, K6MHE

That is very, very interesting Danny. I appreciate the work you did
and the beautiful pictures! I see your conclusion that the antenna
must be at least a physical quarter wave on 40M and that matches
exactly my sentiment (if not experience or knowledge) about building
such a vertical.

Do you have any pics/docs of the telescoping 35-foot whip? I've gotten
pretty good with building wire antennas and hoisting them up into the
sky but have to plead massive ignorance when it comes to whips etc.
PY1BEK's lightweight tower config looks pretty sweet though!

Tim.

On Fri, 31 Aug 2007 13:02:24 -0700, Tim Shoppa
wrote:


That is very, very interesting Danny. I appreciate the work you did
and the beautiful pictures! I see your conclusion that the antenna
must be at least a physical quarter wave on 40M and that matches
exactly my sentiment (if not experience or knowledge) about building
such a vertical.

Do you have any pics/docs of the telescoping 35-foot whip? I've gotten
pretty good with building wire antennas and hoisting them up into the
sky but have to plead massive ignorance when it comes to whips etc.
PY1BEK's lightweight tower config looks pretty sweet though!


As explained in the text the minimum ¼-wavelength for fourty-meters
was to achieve a good match on both bands. Any length from about 33'
will 35' should be fine. There is on real advantage in having it
physically resonate.

That mast is an old military surplus item. I doubt if you could locate
one today. They were used on PT boasts and some Coast Guard cutters
from WWII through the 1950s. Today your best bet would be to use
aluminum tubing or wire strung alongside one of the telescoping
portable fiberglass masts on the market today.

73,
Danny, K6MHE

In message , Danny
Richardson writes
On Fri, 31 Aug 2007 13:02:24 -0700, Tim Shoppa
wrote:


That is very, very interesting Danny. I appreciate the work you did
and the beautiful pictures! I see your conclusion that the antenna
must be at least a physical quarter wave on 40M and that matches
exactly my sentiment (if not experience or knowledge) about building
such a vertical.

Do you have any pics/docs of the telescoping 35-foot whip? I've gotten
pretty good with building wire antennas and hoisting them up into the
sky but have to plead massive ignorance when it comes to whips etc.
PY1BEK's lightweight tower config looks pretty sweet though!


As explained in the text the minimum ¼-wavelength for fourty-meters
was to achieve a good match on both bands. Any length from about 33'
will 35' should be fine. There is on real advantage in having it
physically resonate.

That mast is an old military surplus item. I doubt if you could locate
one today. They were used on PT boasts and some Coast Guard cutters
from WWII through the 1950s. Today your best bet would be to use
aluminum tubing or wire strung alongside one of the telescoping
portable fiberglass masts on the market today.


I haven't done this myself, but I believe you can also use this
technique to persuade a 10m (well, originally CB) halfwave 'Silver Rod'
antenna to work on 20m. You first have to change the feedpoint to the
coil in the base from a tap (typically 1 turn up on a 4 turn coil) to a
simple series inductor. This involves simply lifting the end bottom of
the coil from ground. The 10m matching then becomes an L-match from low
Z to high Z (instead of tapping into a parallel-tuned circuit). The
length of the antenna will then probably need some adjustment.

On 20m, the coil is essentially a just bit of additional loading at the
base of a quarterwave. And you will, of course, need to add some 20m
quarterwave radials. The SWR may not be fantastic, but, if the coax
isn't too long, it can usually be dealt with a tuner in the shack. I
understand that you can also force it to work on 15m.
--
Ian