The St. Louis Vertical Revisited by Dave Gauding, NF0R
Here are a few ideas and observations on the St. Louis Vertical:
Electrical continuity using the Radio Shack 300 ohm in-line connector and the light-duty twinlead leaves something to be desired. The #24 guge stranded wire is serviceable but just doesn’t hold up in repetive use.
Builders may want to omit the in-line connector and convert the original radiator to shorted twinlead or even a heavier stranded wire. Bandwidth is slightly less for the latter though there is no noticeable difference in overall performance. The second alternative is cheaper, lighter and offers less sail area to catch the wind.
Eliminating the in-line connector means that the short at the top of the original twinlead radiator must be relocated to the top of the coil. Add a solder lug termination there and attach an alligator clip to the new radiator to complete the conversion.
The spike can be converted to a sharpened wood screw threaded into the post. It is then removable and perhaps a little safer to handle. The standard spike could be socketed in a slightly oversized hole and stored separately until needed. The post can be broken down into a small package. Slice the wood into several sections, install dowels between the sections and re-assemble when needed. Backpackers wishing to maximize available space might find this variation useful.
The shortened twinlead radials can be replaced with inexpensive speaker wire. #20 stranded seems to work okay up to 50W output. However, a twinlead radial, radiator or feedline offers certain other advantages. The material coils easily and can be stored in a round plastic food container. The twinlead then unwinds to fit snugly in the available space. Coils can be stacked vertical without tangling by separating the radials with cardboard disks.
The radials can be omitted for casual operating where a lower angle of radiation is not required. Setup and breakdown times are considerably reduced. See options under “Feedline”. The ground bus can be omitted by attaching the radial’s alligator clip terminations to each other and then to the balanced feedline.
For a traditional ground plane installation a simple balanced feedline fabricated from #20 stranded speaker wire laying on the earth seems to work well. The antenna will also give good service with a classic zepp feed using the same speaker wire. A tuned coax feedline allows the antenna to serve as a simple random wire vertical. The shield is floated and serves as an elementary RF ground.
Despite what local sporting goods dealers may claim the “Sunny Day” SD-20 collapsible fishing poles are still cataloged and distributed by South Bend. For a dealer near you or a mail order source see the South Bend web page at http://www.south-bend.com or call 708-564-1900.
From KC0PP: Replace the monofilament line and fishing swivel with a round solder tab sized to slip over the rod’s tip section. Select the tab so a friction fit suspends the radiator approximately 4-6″ from the top. Bend the tab over 90 degrees to streamline the installation.
From KI6DS: When fabricating the notched radials cut only through the rubber covering the stranded wires. Then carefully remove the exposed strands with needle nosed pliers leaving behind the rubber insulation. This modification leaves the insulation intact and produces a much stronger radial. Be sure to cover with black electrical tape or appropriately sized shrink wrap.
From KI6DS: A pole labled as “Black Widow” is suggested as an alternative to the specified South Bend product. In addition to the trade name this is marked “Made in China”. It is reported to be a direct copy of the SD-20 pole.
This is a reprint of an article first published in the Peanut Whistle, Sept. 96 issue, journal of the St. Louis QRP Society. Permission is granted to reprint in non-profit journals as long as the Peanut Whistle and the Author are given proper credit.
Resonance on the St. Louis Vertical by Grover Cleveland WT6P
I spent the afternoon assembling the St. Louis vertical. I discovered what others had reported – the antenna is resonant at about 5.6 MHz. So I started pruning to achieve resonance at 7.040 (or thereabouts) using a loosely coupled GDO. The antenna had four radials as specified in the instructions
The final results of all this were that I threw away 26.8 feet of the original 50 feet specified for the loading coil. The final coil wind goes like this: nothing for the first 1.5 inches above the end cap, 13 inches of tightly wound twin lead, the remainder of the twin lead spread evenly up to the top of the first section. The upper wire is a single strand of 18 ga. about 17 feet long spiralling gently down from a solder lug at the top. It is resonant at 7.040 MHz.
Your mileage may vary but I can assure you that fifty feet of twin lead is far too much to begin with. Start with no more than 35 feet or so and begin pruning. Use an analyzer or a GDO.
This is a great antenna! It is going to be heading up to the Sierras next weekend for a “field” test. I am very pleased with the design. Thanks are due to the St. Louis QRP Club for the original work and to Norcal for the current marketing effort. You will love this antenna.
73 and cheers,
Additional St. Louis Vertical Stuff by Dave Gauding, NF0R
Here Are A Few Ideas And Observations On The St. Louis Vertical
Here are some responses to questions about the twinlead coil in the SLV……..and some other stuff!
Twinlead is used in the coil for several reasons. The material is self-spacing. When butted together it produces an inexpensive symmetrically spaced coil which radiates. It handles QRP power nicely and is very durable after installation on the rod base.
Several textbooks note that when parallel conductors are shorted there is some electrical advantage over a single wire, i.e. slightly increased bandwidth. However, the SLV was designed as a center-fed mulit-band antenna for 10-40M, a “seven-bander”. The tuner automatically takes care of the bandwidth situation.
With this in mind the shorted twinlead coil in the SLV is used primarily for mechanical reasons to add strength to the solder joint between the wire and solder lug. The earliest version of the antenna used shorted twinlead for the radiator as well as the coil. A “more is better” gear was engaged at that time! The follow-up to the original article now posted on the NorCal web page covers conversion to a single wire radiator. This mod is simpler, cheaper and seems to perform well by comparison.
For those experimenting with mono-band SLV’s the shorted twinlead should be a definite plus in the bandwidth department. Given the incredible number of portable 40M QRP rigs now in use a design mod optimized for this ban will surely be well received.
The twinlead specified for the prototype places approximately 51′ of radiating conductor on the base of the collapsible pole. The vertical radiator adds another 16′ for a grand total of 66′. Therefore the antenna offers a desirable half-wave on 40M, a full-wave on 20M and so on. It’s almost as if the South Bend SD-20 and Black Widow poles were designed with portable QRP enthusiasts in mind. In any case, we’ll take it!
I had no idea that the dimensions would work out so conveniently until after the prototype was on the air. Initial testing disclosed two important facts! First, locating the SLV’s resonant point with the tuner on the seven bands was a time consuming process. Second, bandwidth at resonance is rather generous including 40M. In my experience the existence of both of these conditions predicts very good news when it comes to center-fed antenna performance.
One reason the SLV works as advertised is full electrical length and plenty of it. And, the spaced coil radiates effectively along with the single wire radiator. Veteran “slinky” ops already know it is possible to work stations with just the coil. I have done this on 40M on two occasions though these exercises were not intentional!
The antenna works on 80M as designed but has not been touted for that band. A tuner will resonate the existing 66′ of conductor which is, of course, a quarter-wave on 80M. I have worked a few stations there to satisfy my curiosity. This was during the summer and more so an experiment than a practical demonstration. Perhaps others will carry this project a little farther now that winter is approaching.
Optimizing the antenna for 80M might include a full half-wave of speaker wire or zip cord for the coil. The additional electrical length will be helpful. Complimentary radials are in order as well. How such enhancements will impact performance on higher frequencies remains to be seen.
There have been many questions about the radials as specified. Three radials seem to work just fine. A lot of early testing was done with only two. If “DX” is a primary interest then by all means add more radials!
The key word here is “logistics”! The SLV was designed first and foremost as a portable antenna. The questions then is how many radials does the portable op wish to install and then un-install at the operating site? As originally specified an SLV can be on the air in less than four minutes without working up a sweat. Builder’s choice applies here!
Radial alternatives for 80M? Even a “shortened” radial has a big footprint. Try “slinky” toys very close to their full extension. There are three retail sizes but the classic baseball-sized coil does a nice job on 80M. Adding at least one of these coils to the original radial system is helpful.
No radials? Sure, why not? A classic zepp feed works well or try a tuner coax feedline with floating shield. Note that the latter does not require a tuner for balanced line! Either option works okay for casual operations and cuts set-up/breakdown times to almost nothing. For a little portable QRP at work during the lunch hour or in the evening after dinner this is the way to go. The angle of radiation is probably awful but the op at the other end won’t know the difference.
New mods reported
From KF9XY: Relieve the wooden mounting post slightly on a lathe to position the rod’s base several inches above the earth. This keeps sand and mud out of the screw cap threads. It also helps insure the feedline conductors remain well separated. Three screws spaced equidistant around the post will accomplish the same purpose.
From KA0JWO: Build several mounting posts to respond to conditions at different operating locations. A sharp, thin spike is better suited to hard ground. A longer, heavier spike such as a gutter spike is appropriate for soft earth or sand.
I’m working on an “SLV Vertical Beam” and an “Elevated Mount for the SLV”. These are fall and winter projects for the OM unless someone beats me to it!
I hope you will find this follow-up helpful. Please keep your comments and mods coming.