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Antennas for QRP

Created by Norman Sullivan, NZ5L on 2023-05-20


"Editor's Note: Due to the popularity of some of eHam's older articles, many of which you may not have read, the team has decided to rerun some of the best articles that we have received since eHam's inception. These articles will be reprinted to add to the quality of eHam's content and in a show of appreciation to the authors of these articles." This article was originally published on: 12/12/2001

Antennas for QRP

As a confirmed QRPer, I have given a lot of thought to the subject of antennas...


My conclusions should be of interest to those planning QRP operations, as well as those of us on limited budgets...


In the first place, despite the obvious advantages, I am going to rule out expensive beam and tower combinations. If you are seeking to save money by building a kit you probably aren't going to spend thousands on a 50' Tri-ex tower and TH7-DXX combo with heavy-duty rotator! That leaves two basic choices - vertical types and wire types...


To me, the first requirement of a good QRP antenna is efficiency, i.e., most of the RF fed to the antenna should be radiated. Unfortunately, most verticals are not renowned for their efficiency. Their short physical length, as opposed to the desired quarter-wave electrical length, almost guarantees a low radiation resistance, which, in conjunction with typical ground losses, adds up to net efficiencies of 10-40%. Not a big deal if you are operating 150 to 500 Watts but not satisfactory for the QRPer. A very extensive radial system or an above-ground counterpoise would improve things, but the best that can be obtained is about 60-65%. (Some alternative designs - mainly some GAP models - theoretically exceed these limitations. If you can afford their entry price - then try it, by all means. My solutions tend to cater to the less well munificently endowed.)


This leaves wire antennas, a field in itself. The operator who puts up a five band trap dipole at twenty feet and then laments about the poor performance will think: "Oh, I've tried that - it was a bummer", or some such sentiment. In fact, even a standard dipole will give a good account of itself if situated in the clear and at least a half-wavelength high at the operating frequency. (65' or more for 40 Meters, 35' for 20, etc.) One way to increase the gain somewhat is to go to a full-wavelength dipole. Use the standard formula, and place the center insulator 1/4 wavelength from one end. The center conductor of the co-ax should be connected to the 3/4 wavelength side. By measuring carefully, a very low SWR can be obtained so no tuner is necessary. Also, if erected as an inverted vee, the long leg can be oriented towards the area of interest. If the high point is your chimney, a short pole can be bracketed to it to give a typical height of 30' or so, excellent for 21 Mhz and above and useable on the 20 Meter band. Total length of the antenna for 15M is about 44', for 20M about 66'.


Another favorite of mine is the Extended Double Zepp, or EDZ. This dipole is based on a 5/8 wavelength per side and gives an honest 3Db gain broadside over a standard dipole. (As such, setting it up as an inverted vee would negate the broadside gain and result in a more omni directional pattern.) An EDZ for 20 Meters would be about 84' long, and could be used on 80, 40, 20 and 10 Meters, if fed with balanced line and a tuner. Some feed line lengths work better than others; I've had good luck using a 42' length of 450 ohm ladder line going to coax via a good 1:1 balun. The performance on 20 justified the trouble of putting it up and gain on 10 (Cloverleaf-style at an angle to the wire) was also pronounced. On 80 and 40 it performed as a dipole. Try to put it up at least 40'. Trees make good supports and will not substabtially degrade the tuning or performance. Use insulated wire if it has to run thru any branches - the insulation won't affect the tuning greatly but direct contact with leaves would!


Another favorite of mine is the Inverted "L". When discussing an inverted L, most hams think of 160 Meters, where it is a primary choice, but it also works very well on shorter wavelength bands, providing a good combination of vertical and horizontal polarization. I will suggest a length here of 165', which equates to 5/16 wavelength on 160, 5/8 on 80 (very effective - think 2 Mtrs!) 5/4 on 40, and 2 1/2 wavelengths on 20, performing as a long wire. It could also be used on 30 Meters. Ideally, an automatic wire tuner could be placed at the base of the wire and remotely tuned, but a common L network could conveniently be used. For operation from 80 and above I would recommend an inductance of 10 uH and a capacitance of 250 pF. A surplus roller inductor works very well and I've seen a few motorized inductors selling for $10-$12 at hamfests! MFJ has been selling a low priced L tuner for years and they are abundant on the used market (tip-if buying used check it out, it may need a repair). For Northeast winters, the tuner should be placed in a waterproof plastic box. Fine tuning at the feed point is really not necessary if you also have a tuner in the shack. The remote L could be thought of as a pre-tuner. You may well find (as I have) a setting that enables better than a 3:1 SWR on as many as three bands, the reactance can be tuned out from the comfort of your hamshack. As with any single ended antenna, a ground connection is required, but the radials (on or in ground) don't have to be resonant or straight and as few as 8 will give good results, as the radiation resistance will be much greater than ground resistance. A copper ground rod makes a convenient tie point, and copper braid can be run from there to the ground post on the pre tuner.


At this point, it is typical to say something like "I busted a pile-up on 20 SSB for 7P8LO with 250 mW" or some such. Well, not exactly, but I can predict that you will be within 2 "S" units of the typical station and will be able to work 80-90% of all the stations you can copy.


For me, it has made QRP operation enjoyable and fun.


Try it, you'll like it.


Good luck.



Antennas for QRP
1. op assumes qrp antenna at qth
2. my first antenna was 23' horizontal in an attic, 45' vertical down 45' of a townhouse... worked the world using $5 worth of wire
3. qrp/p? an end fed half wave or random w/a tuner up a fishing pole works, but a mfla allows multi-band ops with no hassle
4. OMs of yesteryear didn't sweat swr, but today's rig are not so rubust (well, not all, considering the tx-500 or penntek tr-35)