Looking at Eznec models of my 7/21 MHz vertical dipole

My vertical dipole for 7 MHz appears to work well for both 7Mhz and 21Mhz. (ref. My earlier posting). For 21MHz the dipole is a 1,5 wavelength dipole and that made me wonder how the elevation diagram looks like...at least in theory.

Eznec can be downloaded from http://www.eznec.com/. The free demo version suits well for this kind of simple antennas.

The Eznec model for the vertical dipole for the 7MHz band.

The picture above shows the Eznec model for the vertical dipole, using it on the, initially intended, 7MHz band. The elevation diagram is quite ok...If I could raise the antenna higher, the elevation angle would decrease, and the antenna would be better suited for DX working. As comparison I checked, how the antenna would perform, if it would be installed horisontally instead, keeping the feeding point at the same height:

Horizontal dipole for 7MHz, at a height of 10m above ground.

Well...not at all suitable for my DX intentions! What if I could raise the horizontal dipole to 20m?

Horizontal dipole for 7MHz, at a height of 20m above ground.

Well...better, and but the elevation angle is still higher than for the vertical dipole. And I'm interested in DX QSOs. Note that I don't consider directions (azimuth diagrams) here; vertical antennas radiate in all directions and horizontal antennas have direction-specific gains. My interest is to get antennas that radiate almost everywhere. North directions are not that interesting, which makes me think of slopers in my next experiments.

What about 21MHz?

This antenna tunes easily into 21MHz, because it's exactly 1,5 wavelengths on that band.

The Eznec model for the vertical dipole for the 21MHz band.

As seen in the elevation diagram in the picture above, there are two lobes,  at angles of 11 grades and 42 grades. The 11 grade lobe is not bad (and might explain my DX QSOs on that band), but it's 7db weaker than the 42 grade lobe. What if I put up a real half-wave dipole for 21MHz, so that the top is at around 22 meters? Here is it according to the Eznec theory:

For comparison, the elevation diagram for a real half-wave dipole for 21MHz, where the top is at a height of 22 meters

Wow, a nice 9 grade lobe! At least in theory. Yes, why not, since at these heights we are quite many wavelenghts above the ground. I'll certainly climb up in the three and try that some day! Why not install similar antennas for each band? Do they interfere badly with each other? Hopefully not. I have other tall trees in that case :-)

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?