Monday, August 8, 2011

'Nother Awesome Portable Antenna Design - The St Louis Vertical

Whereas the W3EDP is my favorite antler, this one here is probably the one I use most often when I am in the field, mainly because it's so easy to deploy.
This is an antenna I first saw at a NoGA QRP meeting in Atlanta, when I first saw it, I thought to myself "Well ain't this the Bee's Knees."
The antenna is called a St Louis Vertical, named by the St Louis QRP Society, and enshrined at the American QRP Club.
This antenna is:
  • Portable
  • Easy to build
  • Easy to find the parts to build it!
  • Cheap (!kinda, not nearly as cheap as it was!)
  • Multi band
All of which are criteria I value in a homebrew antenna design.  It's best in the 40-10 meter band range, and I have used it on 40, 30, 20, 17, 15, and 10 with good results.  There are however, some caveats, and I would like to relate to you my experiences with it.

Take a good long look at the antenna, and you will understand that it is meant to be a bottom loaded 'short' vertical.  Here's the math though:

1. Do the Math, 1/4 wave vertical on:
40m: 33.29' (7.030 MHz)
30m: 23.15' (10.106 MHz
20m: 16.64' (14.060 MHz)
17m: 12.93' (18.096 MHz)
15m: 11.11' (21.060 MHz)
12m: 9.40' (24.906 MHz)
10m: 8.34' (28.060 MHz)

The only bands that this antenna is short on are 40, and 30M!
What this means is that the antenna will actually be LONG on 20M and up.

Long antennas get funny radiation patterns, especially as the frequencies approach multiples of wavelengths. Basically what happens is the signal stops shooting out (perpendicular to the orientation of the antenna), and starts shooting up. When the antenna is horizontal, that's not an issue, but when the antenna is vertical, it basically means the signal gets shot straight up into the air. Not desirable!
One simple solution to this problem is to use a shorter wire, or leave out the bottom loading coil. It's pretty easy to take a 20' fishing pole and make a quarter-wavelength long vertical out of it for frequencies greater than 14 MHz.

What I need to do to make it better on the higher bands, is find a way to measure the inductance of the coil, and working from there calculate a whip length, and adjust. Hmmm, the coil body is so long, that I have a feeling that trying to use the time honored method of measuring the resonant frequency when paralleled with a known capacitance value could lead to screwy values.  Maybe I need to measure series resonance instead...

I'll take anyone's help here.

Another thing that is slightly buggy sometimes, and can cause you no end of trouble if you aren't aware, is that best performance is obtained when you keep the feedline off the ground.  If the feedline can dance around, then you will notice strange SWR behavior.  I suspect that there is a large amount of signal loss to the ground if the line is allowed to rest on it.  I know signals sound louder when the line is elevated in some fashion.
The St Louis Vertical, took apart, and ready for transport:

Let me explain the parts:
You see in this picture, a "Sunny Day Fishing Pole" that's the brand name I believe. It extends to twenty feet.  Wrapped around the fishing line holder on this pole is enough stranded hookup wire to go from the top end of the coil to the top end of the fishing pole, around 16 feet.  It connects via an alligator clip. The four bundles of wire are the radials for the antenna.  They are made of 3 conductor antenna rotator wire, such as you find at Radio Shack.  The radials are made by attaching all three wires to a ring terminal connector and then nipping out an inch of the individual wires at various lengths along the way, making the longest 1/4 wavelength on the lowest frequency. You should apply electrical tape where you nip the wires to keep water out of the wire, and to keep the wires from pulling apart on a snag.  These wires are cut for 40, 20, and 15 meters, but they'll work 40-10 no problem.  You're using a tuner anyway!  The long spike is used to hold the pole up.  I use a hanger on the spike to keep the pole off the ground, and to also serve as a common lug for the radials.  The feedline is 300ohm twinlead.  The black alligator clip goes to the ground side, the red alligator clip goes to the radiator side.  The banana plugs go to my antenna tuner, usually an Emtech ZM-2.  I love that little thing!

I'm going to try something different with this antenna, namely, I'm contemplating elevating the antenna in some shape or form and making a permanent  antenna out of it.  This version of the SLV is traditionally fed with twin lead so that you can use a tuner to make the antenna work.  I might just keep that feature, maybe use the more robust wireman feedline, or something.

There is another version of the SLV out there, Here's a link to a "For Sale" article about it.  It's a pretty good little antenna too, a lot of hams on the old QRP-L reflector had one back around 200-2001.  I actually bought one from a guy on QRP-L in late 2000, but had the loading coil stolen from me before I had a chance to use it.  It was right after I got my general ticket too.  I had all sorts of plans in my head about going camping with it.  I bought some aluminum ground wire to make a new coil, but haven't made the time to do that yet.  There's always a project going on at my shack!

I received this email from Dave, NF0R, the designer of the SLV.  He posted it to the QRP-L newsgroup, and I have reproduced it here with his permission:

Hello GB,

It appears you are having fun with the SLV. I enjoyed reading your comments and observations on the Blogspot over coffee. I'm retired now and have time have time to put a reply and update together this morning
I designed the SLV (1996?) to be a quick, lightweight and inexpensive vertical. It was for folks wanting to work a little portable QRP, make a few contacts and go back home. No heroics! Just enjoy radio in a portable setting without spending time finding a suitable spot for a wire antenna and still more time installing it. To that end I made it multiband with the continuous coil and a tuner to take advantage of propagation. It's no fun going out with a monoband homebrew rig and finding no signals on your band of choice. I got the idea for using the collapsible fiberglass pole from W7ZOI in Solid State Design.
Referring to your Blogspot you can use coax if you wish to avoid SWR issues associated with wet twinlead. Ditto for stiff line that tends to move around. But It's not a big deal - just tweak the tuner and gone. You can use twinax or parallel runs of RG-58 or even RG-174 if you wish. High-performance with a compromise antenna is always a bit of a battle. That said I tend to favor convenience for portable and working someone (anyone?) instead chasing
the coasts or DX.

I'm not much on elevating the twinlead feedline. As a matter if fact I may have not even tried that experiment. An elevated balanced feedline could add horizontal polarization sort of like an up & outer. But being so close to the ground I doubt it's practicality except for NVIS and prefer to rely on vertical polarization.

I have no idea what inductance the coil measures. It's really a moot point when feedling with balanced line. I filled the base of the SD-20 pole with twinlead because I had enough line in the Radio Shack package to do that. The wet thumb and/or more-is-better school of engineering if you will. It also allowed me to work 80M with the SLV a few times though I can't recommend
The W6MMA wire coil is certainly more efficient due to it's elevated location. However, that particular product is not something you can duplicate without equipment and expertise. You can duplicate it's efficiency by winding a bare wire coil on a section of a child's round float tube and slipping that on thepole. That excellent and elegantly simple design was created by N0TFI (now AE0CW). His coil is a personal favorite of mine. It's real ham radio!
Note that with any tapped multiband coil you may have to swap out upper radiators when some bands change. This is not an issue with the twinlead coil. I prefer to operate when running portable and spend as little time as possible installing and/or adjusting the antenna. My design limit for installs is five minutes tops! Lately I have been running portable with a St. Louis Micro Loop where the set-up time is thirty seconds tops or about the time it takes to walk from the car trunk to a picnic table. For that and other reasons I really enjoy fooling around with that antenna. 
If I decided to re-do the SLV today I would use N0TFI's excellent homebrew coil mounted head-high for convenience. Setting aside most mono and dual-banders today's QRP xcvrs by and large feature on-board tuners. You can get a fairly close match with taps on the coil and touch up the match with the automatic tuner so the rig sees 50 ohms.
My other option would be to homebrew a tapped air-core St. Louis Coil for the SLV which I have already done in multi-band and mono-band versions. These elevated configurations produce very efficient antennas. I also use my version of W6MMA's built-up SLV mount. It employs a pcb feedpoint disk for attaching St. Louis Radials and protects feedline
connections in wet weather.

There were no integrated tuners in commercial QRP xcvrs when the SLV was designed. Hitting tap points on a coil when portable is inconsistent The antenna was intended to be erected in any number of locations and each site is a different RF environment. Having said that there may be
some who still enjoy running back and forth between the antenna and the rig to get a low reading on the SWR bridge. For that and practical reasons I like to keep my portable vertical feedlines short - typically 12-18 feet depending on the lowest design band.

I increased the original SLV's folded radials from three to eight for better efficiency on 30M and 40M. I dropped the notching after several EE's kindly took time to explain how ground proximity affected the resonance of radials laying on the ground. Those folded radials evolved into St. Louis Radials which use ribbon cable. They put a lot of wire (metal) under the antenna which seems to be helpful judging from several field strength experiments. However,
the main reason for their existence is to minimize the hassle with single wire radials and those annoying tangling problems out in the field. Grrrr!
Being fairly supple the stranded-wire ribbon radials are much easier to store, carry and deploy than twinlead. On the trail I coil in groups of three and carry in a round rubber container in the backpack. Otherwise I gather them right below the alligator clips, lower into a cloth bag and
store in the car trunk.
I actually use 1/8th wave radials these days for my portable antennas including the St. Louis Vertical, St. Louis Vest Pocket Vertical, St. Louis Express and several more unpublished designs. These short radials work remarkably and were suggested to me by KK6MC, Jim Duffy, a.k.a. Dr. Megacycle'

I have no qualms running a 10' tall SLVPV on 30M and 40M for chasing DX or milliwatting with eight to a dozen abbreviated ribbon radials under it. For all of these portable verticals the footprint ends up being very small.
Thanks very much for your post about my old SLV. I certainly enjoyed reading it and appreciate your time and effort. I hope you find my comments helpful.
Best regards,

Dave Gauding, NF0R

Ain't this a fun hobby?


Anonymous said...

Check QRP QUARTERLY Oct 1999 & Jan 2000.

Brian said...

Where can i find instructions on building the st louis micro loop?