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 at http://svc.cc/antena.html

Tim N3QE

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

You could use a trap instead of a relay.

Another approach would be to chose a fixed vertical height
to give you a manageable feedpoint impedance on both bands.
For instance, a 25 foot vertical requires an easy to implement
base network on each band. EZNEC says the feedpoint impedances
are 17-j130 on 40m and 200+j320 on 20m, both easily matched
using common network components (or an autotuner).

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

Ground proximity detunes the radials so length is only of
minor importance. 40m ground radials work fine on 20m.
--
73, Cecil http://www.w5dxp.com

On Aug 31, 10:13 am, Tim Shoppa wrote:
I am postulating a full-height quarter-wave 40M vertical that is
switchable (relays halfway up?) to a 20M vertical dipole.


Myself, I would base feed for both bands. Say if you used a
"gamma loop" matching scheme for 20m.. Use a relay/relays
to cut the connection from the inductor to the radiator. With
multi-pole relays, a set could be used to connect the feed
directly to the radiator when the inductor is disconnected.
Of course, it would need to be weatherproofed.
I used a similar setup to switch my 40m elevated GP to
a 5/8 GP for 17m. If I left the inductor used for 17m inline,
my resonant 40m frequency was about 6.900 or so.. So I used
a 24v relay to disconnect and bypass that coil for use on 40m.
You could also use a regular coil for tuning the 1/2 wave,
but I prefer a single turn which is tapped at the best point.
If a cap is needed, and it probably would be on 20m, I use
a length of coax. "open at the far end, not shorted".
One thing about splitting for a 1/2 wave.. You gotta have
insulators in the middle of the mast, and then rig a way
to bypass for 1/4 wave use. I guess it's do-able, but I'd
just as soon use a one piece radiator and feed at the base
for all bands. BTW, with even more rigging, it could be used
for 3 bands, if you added 17m running as a 5/8 GP.
All you need for that band is a series coil at the feedpoint.
I used 24v fan relays usually used for switching fan motors,
and ran wire down to a 24v transformer here in the shack.
I'd unplug it for 17, and plug it in for 40m.
BTW, yes I did also unplug and throw that wiring outside
in bad T-storms.. Same as my feedlines.. :/
That wiring ran from the feedpoint at 36 ft, down the mast
to the shack. Could carry a hefty pulse if I took a strike
same as the feedlines.
The radials should be no problem. IE: 1/4 WL for 40m.
I think most BC users that run 1/2 waves, use a field
of 1/2 wave radials.. So, should work out ok I would
think. Besides, any radials on/in the ground are going to
be fairly detuned anyway. A 1/2 wave really needs no
radials, except that ground losses will occur when
ground mounted, same as any other antenna. But
they will be less than a 1/4 wave used on the same
ground mount, and same band. The set of 1/2 wave
radials should keep that loss fairly low. Will be
better than the 1/2 wave ground mounted with no radials.
MK

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 at http://svc.cc/antena.html


Take a look at this.

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

73,
Danny, K6MHE

lots of problems with that... how do you decouple the 20m midpoint feedline
from the 40m vertical? how do you get the 20m feedline up there without
messing up the 40m performance? where and how do you put and connect
radials for 40m?

a slightly more practical thing would be to have a relay half way up to cut
off the top section to turn the 40m vertical into a 20m vertical... but the
best and most common way is to put a trap at the mid point to cut it off
automatically on 20m... add more traps, you get more bands.

"Tim Shoppa" wrote in message
ups.com...
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 at http://svc.cc/antena.html

Tim N3QE

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.

Dave wrote:
lots of problems with that... how do you decouple the 20m midpoint feedline
from the 40m vertical? how do you get the 20m feedline up there without
messing up the 40m performance? where and how do you put and connect
radials for 40m?

a slightly more practical thing would be to have a relay half way up to cut
off the top section to turn the 40m vertical into a 20m vertical... but the
best and most common way is to put a trap at the mid point to cut it off
automatically on 20m... add more traps, you get more bands.

"Tim Shoppa" wrote in message
ups.com...
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 at http://svc.cc/antena.html

Tim N3QE


assuming the only problem is the switching a relay could be arranged
to extend the antenna for 40M operation if it was air activated. I
might try making the tower so it was 1/2 wl end fed antenna on 20m and
1/4wl on 40. If you can insulate the base from ground that should be
easy enough to do.


Jimmie

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

With a tubing vertical you can run the coax up the inside of the lower
half to feed the dipole and use relays to switch between base feed and
center feed for changing bands... GAP does something similar with
traps (after assembling a GAP for a local ham I am not a fan of GAP
antennas, their basic idea is sound however) You will need a choke
coil right at the base and probably should bury the feedline into the
soil as deeply as you can trench it - at least 3 or 4 feet - and have
another choke a quarter wave back from the feed point... Lots of
work, not much extra cost... Should work OK...
Other way to do it (and my preference) is think like a Ringo Ranger
and stay with base feed... Insert a half wave, parallel feed line,
phasing section at the mid point and get the benefit of 2 quarter wave
in phase radiators on 20 meters... Short this phasing line out for 40
meters.. Again, run the relay wiring inside the lower tube...

denny - k8do
In business as "Solutions are Us" - as long as I don't have to
actually do the work!

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