Tuesday, 12 April 2011

A vertical dipole for 7MHz (and 21MHz)

For a while I have been planning on my next simple antenna project. I have the high trees, that I can utilize, as I did with the folded dipole. About 40 meters from my house there is a high fir tree, maybe around 30 meters high. I'm not afraid of climbing, so I decided to make a vertical dipole for the 7MHz (40 meter) band. Also I hoped that could help me getting a good antenna also for 21MHz (15 meter band), since according to the theory, a half-wave dipole for 7MHz is a 5/8 wave dipole for 21MHz. 5/8 wavelength tune well to 50 ohm impedance, ant at least when it comes to ground plane antennas, 5/8 means a low elevation angle for the radio waves. And this means DX.

Here is the starting point. 
I prefer factory baluns, since they are robust and durable. The antenna wire is plain old phone wire.
A half wave dipole is very simple to construct. IAround 10,5 meters of wire in each leg.
Temporary location at a height of 2 meters.  
I hang up the dipole horizontally at the height of 2 meters, in order to get an initial measure of the SWR, which was 3:1. After cutting, it was around 2:1, so I decided to stop cutting (after the lesson learned with the folded-dipole project). Yes, I had to try a QSO with this temporary antenna installation, the dipole legs hanging indefinitely at 1,5-2 meters. No problems: Europe hears me and I hear Europe. But...what about the DX, which I have never been able to catch on 7 MHz? Up in the tree!
Up in the tree, at around 25 meters. I took a camera with me, in order to document the views from the fir. There...far down, is my excellent folded  dipole.
Nort-west view from the tree. Yes, still some snow everywhere, and the leaf trees seem dead...we are in Finland, right?
North-east view from the tree. In the summer the views aren't this open because of the leaves. Let's see how it affects the performance of the antenna.
When I got the wire in place, the balun was hanging at about 12 meters and the 10 meter legs up and down from there, I was warm and sweaty.

The results:
After the shower I started my radio. The SWR was little less than 2:1. Not much, but anyway I decided to use my antenna tuner. The 40m band sounded crystal clear: almost no noise, only strong and clear CW beeps. And the digimode waterfalls looked brilliant...I compared these to the windom antenna, which is installed at about 8 meters...the difference is remarkable. Boy, I was excited!

There was the Japan International DX contest going on...let's check. I heard many Japanese stations right away. When I swithed back to the windom, the stations went weak again. Well, my antenna looks promising. I answered a CQ from JH4UYB (Japan)  and he heard me immediately! He didn't get my call 100% at the first try, but after a couple of repetitions we could finalize the QSO. Obviously this guy is a big gun, with strong power, giant listening and sending antennas, but anyway: my wire vertical dipole works can perform DX. the propagation conditions were not superb at all, so there is still lot to explore with this installation. (Later I might arrange the antenna as a sloper, or even horizontal dipole...let's see.)

Also the Europeans are now delight to work...my fellow hams report strong and clear signals and I hear them clearly. And the waterfall display on the digimodes does not look "noisy" anymore: When switching between the new antenna and the windom, there is a remarkable difference in the noise floor in the specrum view. No windom again on 7MHz, that's for sure!

What about 21MHz? 
Guinea-Bissau and the States were worked almost immediately. Just a few QSOs made, but this looks promising. Also, I'd like to check this 5/8 wave stuff with Eznec simulation: what were the theoretical elevation angles with a vertical dipole like this?


  1. The antenna wire is touching the tree? I am wondering if the trunk has some influence on the operation. You could also use a guy line between two trees and hang the vertical from the line mid point. A 80 m folded vertical would be also interesting to try. That is the extra wire height is put into horizontal direction at the dipole ends.

    73, Jaakko OH7BF/F5VGL

  2. The wire is about half a meter from the three, between the branches. The tree is in connection with the ground, and an alive tree is quite dense, so it possibly affects the antenna in a negative way. Of course the effect would be more severe with UHF and microwave antennas :)

    Hmm...guy wire. Maybe a stack of verticals hanging from it...too heavy? If I can get wires horizontally between my trees, I might start thinking about horizontal dipoles or loops. I don't have a proper antenna for 80 meters, but I do have space for a horizontal full-size halfwave dipole. For DX working I might consider vertical stuff even for 80 meters, L dipole maybe? And 160m? I am just in the beginning of my antenna experiments.

  3. I modified this vertical dipole, made it an inverted v-dipole: http://oh3ggq.blogspot.com/2011/06/inverted-v-dipole-for-40m.html

  4. Later I realized that the inverted-v (mentioned in the previous comment) outperformed my vertical dipole. Obviously the vertical dipole is not high up enough. The last try, a GP 12 meters up, is even better, and may be my last 40m antenna before the future 3-elemement beam :)

    Here is my antenna "park" as of October 2011:

  5. What final length did you wind up with on your vertical dipole ??

  6. Actually it is exactly 10 meters high. Also the three radials are 10 meters...Optimal length would be a little bit shorter, but SWR is less than 1:1.3 now.