Topband: Short dipoles

[Jim Cain]

Re: Short dipole for 160.

I asked on this reflector for opinions on whether I should extend a 180-foot centerfed to 260 feet. It's inverted V configuration, center at 38 feet and ends at about 15 feet. It's fed with 450-ohm balanced line and I need for it to be my only antenna for 160-10 Meters. It tunes OK on 160 although it's very sharp. The additional 40 feet on each leg of the antenna would be 10 feet or less above ground, and one of the 40-foot extensions would probably end up being a dog leg. The antenna tuner feeds 10 feet of RG-58, to an MFJ W9INN balun box, to the balanced line. I have an earth ground on the balun box.

I know, I know, this is far from optimum, using coax to a balun, but I am renting here and getting the balanced line into the house would be a chore. 

And I really didn't want to buy more wire and then horse around changing an antenna that already works pretty well.

But I did. And Friday morning about an hour before sunrise K5NZ told me I was five dB over S9 in Houston (I am in eastern Indiana). That's with 100 watts output.

I didn't really get any on-air feeling for the shorter antenna so I can provide no empirical evidence that lengthening it to a half-wave makes any difference. But, I think it hears a little better. And it still tunes just fine on all the other HF bands. 

Tom Rauch, W8JI, directed me to his web site, http://www.w8ji.com/short_dipoles_and_problems.htm, "Problems with short dipoles." There I read what I suppose I knew, in my gut, and thought "Jim, you cheap, lazy dog, you have the space, buy some more wire."

I appreciate those who wrote to me with suggestions. As I said originally, even though there is a 40-foot tower here, I don't have the incentive to do an inverted L dedicated to a band I will probably appear on only two weekends all winter. 

I'm passing along the comments received, in case this information might be useful to others.  

Cheers, Jim Cain, K1TN/9 Indiana

    (1) If your only interest in 160m is for an occasional contest, then I suggest putting on the additional wires with a breakaway connector, such as banana plugs.  This way you can hang the wires quickly for the weekend, then take them off when you are done.

    (2) Over the many years I've been on Topband, it has become clear to me that having a full size (around 250 to 260 ft long) is really essential for reasonable performance. While on other bands, people are quite successful with, literally, a piece of wire mounted outside a window into a tree, for example, a shortened antenna on 160 is particularly handicapped for basic efficiency. 

One thing I did at the very beginning was to parallel two antennas with a common feedpoint.

What you need to do is to note the tuner C and L loading points so that that section of wire is loading properly for the upper bands.

Load the longer antenna for 160 separately and make note of those settings. When the two antennas are then tied together, you will know which antenna is loading on which band.

Of course, you can go out and connect manually, or with a relay, the two 40 ft. extensions. At one point, I loaded a 130 ft antenna on 160 with a switch to flip back and forth to the full sized antenna and consistently noted a 20 dB difference.

All my transmit antennas here are fed with the 450 ohm line.

One thing I also used successfully in the beginning was a shortened W9INN antenna with loading coils, or he called them resonactors, and the 180 ft. version worked pretty well. Unfortunately, he was a one man operation and Bill became a silent key earlier this year.

I've also used EZNEC extensively but the data has to be interpreted with a fudge factor but I have found reasonable correlation between the data and performance of antennas in general.

    (3) Yes, it will not hurt to add the 80 feet and make the flat top a bit  longer.  No harm really with the ends parallel [to the ground], as most of the radiating is near the center anyway.  If it messes up any other band (the tuning will change for all of them) why not just connect the ends when you want to broad tune 160?  Success will depend to a great extent on how your tuner reacts to the antenna.

My cloud warmer is a full sized 1/2 wave inverted vee made of open line, one side cut for 160 and one side cut for 80.  Apex is at 80 feet and ends about 15 feet off ground.  Fed with coax, I need to "retune" every 20 khz on 160. It is fine across 3500-3550 or 3750-3825 without need to retune the tuner. I use an MB-5A on this antenna so retuning is only a matter of moving the rotary inductor a bit as I move up the band.  An inverted L beats it consistently for medium/long haul.

    (4) Adding the additional 80 feet to the antenna will make it closer to a half wave on 160 meters and will definitely improve its performance.  The additional length will make the antenna two half waves in phase on 80 which will perform quite well and if you are feeding the antenna with open wire line, shouldn't cause you any problems on the other bands.

    (5) Jim, for your objectives, I'd leave it as is.  Hopefully not a sour note, there are lots of contest stations on from Indiana, but more are always welcome.  

    (6) I have tried 2 x 30m (just about 2 x 90 ft) with 600 ohm feeder and it works just fine 160-10. It will be less then 1 dB down from a 2 x 40m (halfwave dipole) on 160. I see no need at all to make it any longer then what you have. You might check the W4RNL web page, he writes about similar antennas.

    (7)  Let me begin by admiring your humility - especially when I recognize you as a top-notch author of antenna articles in QST and elsewhere. 

But to the subject at hand, I've spent many years playing with antennas on 160 meters and I can offer this suggestion:  Put up an inverted L. Try your inverted Vee, but if you can do comparisons with an adjacent inverted L I'm sure you will be amazed. From your new QTH, coast-to-coast QSO's should be routine, and working some of the big guns in EU (e.g. DF2PY) should be no problem.

(8)  I am truly interested to see what sort of suggestions you receive from the reflector crowd on your antenna. I too am using a sorta inverted V for all Bands that is 89ft on each side of center and fed with 450 ohm ladder line. It is 49ft at the center and roughly 20ft on the ends and is my All Band Antenna!!!! I have been a Ham since 1963 and run either 100W(my QRO) or less than 5W for qrp. This antenna of mine seems to load OK on 160 thru my MFJ tuner,  but apparently no one can hear me calling CQ, or I am not hearing the response!!!!  160 may not be the band to be cq ing on!!??

    (9)  I've been a ham for 43 years in 11 more days! I have used the antenna you described, except mine was 200 ft. long, not 180. You'll not be able to tell any practical difference if you did lengthen it. And if you're a cw op and a tenacious type, you can expect to work some dx too!

    (10) I use a double bazooka antenna that is 40 feet high in the center and 30 feet high at the ends [in upstate New York]. The antenna is 240 feet long. I am on a hill about 1700 feet above sea level. When I moved here [from New Mexico] I had 80 countries on 160 meters and now I have 115 countries. ( using max power except for a couple of countries. I forgot to turn the amp on from the standby postions and still worked the countries...Hi).

I would add the 40 feet to each end.

When I was in New Mexico, I used a double extended zepp (1330 feet long). I made a balun to match the odd impedence to 50 ohms. The center was at 80 feet and the ends were at 15 feet. I was one of the few stations in NM that could get into europe on 160 meters. At the end of a contest (CQ 160), at sunrise, I would run 20 or 30 JA stations.

This antenna also helped me get WAS with 100 watts..

The point I am trying to make is that the height is not as critical as some people think.