Screwdriver Antennas

Overview

Generically, the screwdriver antenna is a vertical with a tuning/loading coil adjuted by a cordless screwdriver. The bottom of the antenna is a piece of tubing big enough to cover the motor, the top is a straight whip. There are a bunch of firms selling plans or antennas, a search for "screwdriver antenna" will turn them up. You'll also see this referred to as the DK3 antenna or the W6AAQ

My Antenna

My screwdriver antenna is a BBC3 from Nott Ltd, purchased from Ham Radio Outlet. I don't claim that it's the best deal, the best constructed, etc., just that it was in stock when the "bug bit". There's a bit of detail on how I mounted it on my mobile rig page.

Here's a link to an article by Ron Nott: http://www.antennex.com/preview/Folder01/screwdrv/screwdrv.htm

I've also rigged up a tripod of collapsible painter's poles to support the antenna for field operations.

Some Measurements

Now that the antenna is installed, I've made some preliminary measurements on it. Overall comment: Tuning is very sharp on lower frequencies (but I knew that going in, what do you expect from a short radiator and tuning coil)

A comment from Nott's writeup in Antennex: Whip length also gets into the impedance matching act as it provides the top loading which reflects back to the antenna input. For 80 meters, a 96 or 102 inch (2.4 or 2.6 m) whip works best in most applications. From 20 meters up, a whip as short as 63" (1.6 m) works quite well and still allows operation in the upper end of the 75 meter band.

This doesn't really reflect what I've found. A short whip (on the order of the 63" mentioned) doesn't really tune 75 meters, and it's very, very touchy on 40. 10 doesn't tune so well, either, but 20, 17, and 15 work fine

I have to say, based on some measurements made in the lab, that the turns ratio probably is around 2:1 (4:1 Z).. but still, with a radiation resistance of 1-2 ohms, that's only 8 ohms into the feedline. You really need something like a 4:1 transformer (16:1 Z)

For a 9 foot (2.74 m) total length, assuming it's a monopole (i.e. really like a 18 foot short dipole), the radiation resistance (using 20* (2pi/lambda)^2 * h^2 ) is as follows: (ignore the spurious accuracy.. )

Freq
(MHz)
Radiation resistance
of dipole
3.66 .885
3.89 1.00
7.22 3.44
14.308 13.5
21.8 31.4
28.5 53.7

The matching box

I made some measurements on the matching box with a high impedance load and into a 50 ohm load. A HP3325 was the source and a dual channel oscilloscope was used to measure the input and output voltages. About 1 meter of RG58 coax on the feed side, another meter on the load side. The data is in an Excel Spreadsheet (screw1.xls - 24 kB) It appears that the turns ratio is 2:1. At low frequencies thats the voltage ratio from input to output. With the 50 ohm load, the effective ratio changes from 1:0.6 (at 1MHz) to 1:0.3 (at 20 MHz), which tracks fairly well with the apparent load impedance (the 50 ohm load reflected to the primary side by (turns ratio)^2).

I've also popped the lid off, and it is, in fact a toroidal core with a bifilar winding. There's no capacitor from input to output, which I've seen on some other matching networks proposed for these sorts of things.

References

B.F. Ansell,III, KI0AR, http://home.att.net/~bfa3/sai.html A matching transformer using 75 ohm coax.. 68 inches at .66 vel factor = 103 inches free space.. 2.62 meters. That's around 1/4 wavelength for 10 meter band, but nowhere near that for 40 meters, etc. However, it does provide some impedance transformation, and the reactive component could be cancelled by the coil in the antenna.

http://www.hamradiotoys.com/dk3.htm - A page showing the DK-3, with an interesting variation on the feed network instead of the toroidal transformer (incorrectly spelled torrid, as in blazing hot, on this website...). It shows an inductor at the bottom, but it's not real clear how the inductor is made, but it's referred to as a Z match.

http://home.gwi.net/~brandall/mara/squelch/jan00.pdf has an article on page 3 describing another matching network technique: a relay to switch between toroidal transformer (good for the lower frequencies where the R is very low) and a shunt C for the higher bands. The article is by W5ES, and here's another link from him: http://www.qsl.net/w5es/installations.htm (this is the "ElectraSlide")

http://www.mindspring.com/~k4poz/page2.htm shows coupling units (toroidal transformer with carefully chosen leakage C?).. No construction details given (obviously, the site owner wants to sell the units, so this is reasonable). Note that this site has annoying (to me) musical background.

http://www.bandhopper.com/page2.html gives some specs for their particular antenna, the form factor of which is somewhat different (and the length doesn't change, being a constant 9 ft). Note particularly the SWR bandwidth, and the kind and length of coax (yep, 42 ft of RG-8X is a nontrivial feedline...), and the fact that they're operating it 18" off the ground (I wonder if they had a stake in the ground?)

SPECIFICATIONS:
FREQUENCY ---------- S.W.R ------------ 2:1 BANDWIDTH
3.660 MHz -------------- 1.05:1 ------------- 50 kHz
3.890 MHz -------------- 1.1:1 -------------- 46 KHz
7.220 MHz -------------- 1.35:1--------------100 KHz
14.308 MHz ------------- 1.0:1 -------------- 2.1 MHz
28.5 MHz ----------------1.0:1 -------------- 3.0 MHz
GENERAL BANDHOPPER MEASUREMENTS The above measurements were made in 1993 using a Kenwood TS-940S transceiver with the BANDHOPPER antenna mounted 18" above ground through 42 feet of RG-8X coax. Your measurements may vary depending upon your particular mounting configuration and how close the coil portion is to near by metal surfaces.

They also have a relay box that switches in 100, 390 or 1000 pF (for 20,40, and 80) in parallel with the feedline, close to the feedpoint with a coax "T". Another control box uses 50, 390 and 1000, switched a bit differently. I kind of like their control box designs, they put the relay which run the antenna motor and switch the tuning components, etc near the antenna, and have a remote switch box to run the relays.

http://www2.cconnect.net/newbernarc/Newsletters/NLApr02.pdf has an article talking about loading/matching techniques by Andy Griffith W4ULD

M0CKE has a bunch of pages describing the mounting, etc. http://www.j1mbo.f9.co.uk/dk3-ft100.htm

http://home.att.net/~DickWD8CEB/Balun.html has a drawing showing how a balun for these antennas is made, with turns ratios shown.

radio/antenna/screwdriver.htm - 26 August 2002 - Jim Lux
significant update with some test data 13 Jan 2003, and again 1 Feb 2003
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