RE: DX & SLV

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From: david gauding (david.gauding@bbs.galilei.com)
Date: Sun Jan 28 2001 - 12:58:32 CST


At 01:43 AM 1/28/01 -0600, you wrote:
>Hi Dave,
>
>I am building the St. Louis Pocket Vertical for travel on an airplane.
>Is your vertical ground mounted?

The St. Louis Express is ground mounted.

> Are your radials on the ground or
>elevated?

Grounded. Though it should be mentioned that the feedpoint of the SLX (or
the SLPV that you are building) is 7-10" high depending upon the
installation. And, the somewhat rigid ribbons slope outwards and downwards
from that point. To complicate things further the proximity of the seven
conductors introduces additional capacitance into the mix. The radial
themselves are suspended several inches above the earth by grass. Finally,
the pvc insulation does not allow them to touch the ground. I have
absolutely no idea what is happening electrically but the SLR's do seem to
work okay for what they were intended to be, i.e. portable and easily
managed radials.

I'm using nine St. Louis Radials with all seven conductors cut to 16.5' or
an eighth-wave on 40M. I converted to this configuration for all my 10-40M
portable verticals at KK6MC/5's recommendation. The same length covers all
the design bands. Jim Duffey feels the minimum is twelve eighth-wave
ribbons. But, I use nine as a reasonable compromise between performance and
convenience in the field.

>Do you use a tuner at the base of the antenna.

No tuner is required for the SLPV that you are building. I tap the
air-wound St. Louis Coil built exactly as described in the article in QRPp.

>I've modelled
>the antenna with EZNEC over salt water, as I use antennas on the island of
>Bonaire. (I've corresponded with you before on this subject.) I find that
>the radiation resistance is ~ 12 ohms when the coil is set to counteract
>the -jX of the short radiator. So, even if it's resonant with the right
>tap on the coil, it still has an SWR of 3:1 with 50 ohm feed. That applies
>only
>if the radials are truly efficient, i.e., not just laid on the ground but
>elevated
>or used over highly conductive earth.

I've never modeled any of my antenna designs but I do put every one of them
on the air and test extensively before committing to an article. Being
decidedly non-technical I have to follow my ears and later page through the
log to judge an antenna's performance.

> With high resistance radials, the SWR
>will
>not be so touchy, but the efficiency of the antenna suffers, as I'm certain
>your know.
>Are you using the SLV radials as you described in QRPp?

The portable folded radials used with the original SLV were the best I
could come up with in 1996. St. Louis Radials were developed later along
with the St. Louis Express project. As a cliffdweller, I was after absolute
convenience in the field and performance after that. Spending operating
time hanging wire in trees or erecting the all-metal Random Wire Vertical
got pretty old after 15 years of that.

I wanted to set-up quickly in a portable location, make a few contacts on
10-40M and go home. After a month of regular visits to River Bend School, I
reviewed the log and decided the little vertical wasn't doing too badly for
what it was and wrote it up for others to try.

>Wonder what your exact setup is. whatever it is, it sounds like it's
>efficient, which is one of the things QRP is all about!

I'm not really sure if the St. Louis Express is efficient in terms that
engineers must necessarily use to measure and define it. But, it is a fun
and useful antenna and meshes with my interpretation of QRP. I'm closing in
on 50 countries worked on 40CW running QRP with the SLX. I probably have
500 plus contacts in the log with the vertical at one watt and that
includes a little casual DX. I didn't get serious about chasing DX on a
low-band until a few months ago. After laying off the stuff for ten years
guess I'm getting hooked again!

The antenna used to work the French contesters last night on 40CW (on a
very good band) is a resonant version of the St. Louis Express vertical as
reprinted in the Summer 1998 issue of QRPp. The original SLX is remotely
tuned between 10-40M and uses a loaded ribbon cable radiator.

Last year, I stumbled on a way to resonate the ersatz radiator on 20M. That
development produced the very portable St. Louis Quickie vertical. The
article with the companion St. Louis Vest Pocket Vertical are in the
current issue of QRP Homebrewer. The SLVPV is the 10-20M version of the
SLPV for 10-40M that you are building now.

The SLX is my favorite portable 10-40M vertical. It is also my current
"winter antenna farm" and a candidate for long-term testing. With this in
mind, I lengthened the the SLX radiator several feet so it resonated
independently on 40M. To do that I essentially doubled the SLQ dimensions.
Being self-resonant the losses through the tuner are assumed to be somewhat
less on 40M than the remotely tuned version. The revised design will end up
in an "slx revisited" type article later this year but the original antenna
still gets the job done.

I believe an air-wound coil radiator is hotter than the loaded ribbon cable
radiator. Also, that the angle of radiation is lower on the more
traditional design for identical antenna heights. I can't prove this on
paper but it is my opinion after many, many hours using such verticals on
the air. And, not being in a life threatening situation I don't think the
differences are all that significant. I worked DXCC and DXCC/QRP with attic
antennas. Talk about attenuation! I mean just how hard is it?<g>

Having said this, right now it's snowing/sleeting lightly and the
temperature is below freezing. Given a choice between trotting outside and
changing the tap on the coil or dialing up another band on the tuner, I'll
take the warm way out.

The SD-20 support for the SLX is fitted to a St. Louis Mount. This is a
modification/development of the excellent base mount that accompanies the
SLV/W6MMA coils. It features a larger double-sided pvc feedpoint disk used
by the SLPV, SLVPV and SLQ to easily take the alligator clip terminations
of the St. Louis Radials.

The radial system used under the SLX these days (and the SLPV you are
building) is as described earlier. Bottom line, the six quarter-wave St.
Louis Radials for 40M used in the original article have been changed to
nine eighth-wave radials.

Have to end with one shot of reality for portable operators. I can
breakdown the the SLX antenna system now in my yard and re-install it
anywhere else in five minutes. Period, no exceptions! That was a design
goal and it was and still is important to me.

On-air performance for the SLX discussed here should be similar in
different locations if the antenna is positioned in the clear. I can't
reproduce the snazzy operating results at home in the summer because of
absorption in dense foliage in our yard and adjacent yards.

Hope this response to your question is helpful and thanks for helping me
pass some time this morning composing it.

Regards,

de Dave, NF0R nf0r@slacc.com

>Thanks,
>Andy W9NJY
>Milwaukee
>
>-----Original Message-----
>From: owner-qrp-l@Lehigh.EDU [mailto:owner-qrp-l@Lehigh.EDU]On Behalf Of
>david gauding
>Sent: Saturday, January 27, 2001 10:49 PM
>To: Low Power Amateur Radio Discussion
>Subject: DX
>
>
>Worked F6IPA on 7.015 at 0436Z. F6KLI and F6CEL a few minutes earlier. All
>contest exchanges.
>
>Band is very good. I'm running one watt with a Argo 515 to a St. Louis
>Express vertical.
>
>
>de Dave, NF0R nf0r@slacc.com


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