Saturday, February 11, 2012

HF-antenna's Vertikal Dipole 10-40m

HF-antenna's Vertikal Dipole 10-40m

Since we're heading to lower sunspotnumbers it was time to consider an antennaproject for the lower HF-bands. We all tend to start with a dipole which we usually can hangup in a decent way from 20 to maybe 30 meters. For 40 and 80 meters the dipole will have a length which becomes difficult to hang up fully horizontal . This is usually the length where some HAMS may come to the very painfull constatation that the lot or available space to play with antenna's should be much bigger (for an antenna freak, the lot should ALWAYS be bigger). Of course we can hangup the antenna in inverted vee, of course we can bend the ends, of course we can use techniques to shorten the dipole. This article and related physicall testing is ment to try to find an alternative for the horizontal dipole. Antenna efficiency, antenna pattern, used space and user satisfaction are taken into account.


We will take an inverted V for 40 meters, an antenna which is widely known amoung HAM's as an allround antenna . Many know what to expect from a dipole, I will try to compare this allrounder theoretical and practical with a shorted VERTICAL dipole antenna which can be set from 10-40 meter.


A well known item by DX-ers are elevation patterns. Due to many discussions about take-off angles, we might assume to easily that we need as much energy at as low as possible angles to work the DX. I believe you need the energy at the right angle at the right time and I don't expect an antenna which radiates straight up to be my main DX tool.

In FIG.1 we see several elevation patterns from wel known antenna's. Pink marked we see the elevation pattern from the inverted-V, which I modelled at a moderate hight of 1/4 Lambda. With this antenna we can see that a lot of the energy is goes upwards to the sky. This will make the antenna a champ for local contacts within the 800 km range. Even at this moderate height, this inverted VEE will need a 10 meter high support and a 20 meter long garden strip!

The black pattern gives the radiation from a short vertical dipole with capacitive HATS and coils for some center loading and exact tuning. At a 24° angle you can see that it gives as much energy as the inverted-V dipole. At lower angles there is little more and at the higher angles you will be below the dipole. Very interesting is to see how the place of the antenna is affecting the pattern, for worst case I assumed AVERAGE ground conductivity of 15 mS/m.

The blue pattern gives you an idea about performance if you can place this antenna in rich soil. I already started to glance after calculating those patterns.

And... if you're near salt water or go to an island expedition, there is absolutely no reason to do much effort in putting up horizontal antenna's, unless you can hang it as high as a wavelength.

The modelled inverted-V antenna is full size and will have a higher efficiency than a shortened horizontal dipole. If we consider a full size 1/4 wavelength vertical we need a 10 meter high radiator and enough radials for low angle radiaton and appropriate efficieny. I will not go into the theory on the amount of radials needed but even if you only use 4 of them you need a big square to install the vertical antenna for 40 meter (we didn't consider 80 meter yet !!). Some HAM's may not see the problem, but in a small country like Belgium a small lot of 800 square meter will be "average" for many HAMS, and of course our XYL don't want us to spoil the whole garden with wires hanging "around". Placing the dipole not horizontal but vertical with cancel the energy which goes upwards and provide low angle radiation depending on the conductivity-properties of the ground in the environment (several wavelengths around your antenna). If you're not in the desert you might have some advantages in using the dipole vertical. A full size vertical dipole for 40 meter will also be about 20 meter high, which might also be difficult to manage. Shortening the antenna will able us to accomodate the antenna. The most efficient way to load an antenna is top loading, if we load the antenna with coils we will have to deal with losses in the coil which can very fast turn into "heat radiators" intstead of radiators for our valuable RF-power. Instead of using the full size vertikal dipole or inductively loaded shorter dipole or monopole we can reduce the antenna size without loosing much gain by using capacitive HATS or a combination of both for lower bands. Hats are symmetrical arrays of wire at right angles to the dipole ends.It is the symmetry which results in radiation cancelling and hence does not yield significant horizontally polarized radiation. Non symmetry will not have much influence on the radiation, but will reduce the operation bandwith with a significant amount. The vertical dipole which we will test can be used with capacitive toploading from 10 to 20 meters, if you add some high-Q coils at the feedpoint you can use it on 30 and 40 meters too....BY SHORTEN THE ANTENNA THIS AMOUNT, WE WILL MAINLY LOOSE BANDWITH RATHER THAN GAIN.

6 uH coils, 11Turns on 50 mm with 4mm² Cu wire. By increasing the winding spacing will increases the resonance frequency. Coil in the serves as B-match.


The bandwith of an antenna is one of the important issues when we evaluate an antenna. I always assume operating bandwith to be 1.5:1 VSWR bandwith, higher VSWR values will require automatic or manual impedance matching.
On 10 and 15 meter there is only a small amount of shorting, so operating bandwith is wide enough to cover those larger bands.
On 20 meter we can easily cover 350 KHz with the shorting HATS.
30 and 40 meter do need the coils placed at the feedpoint, bandwith decreases to about 100 KHz due to the coils, but this isn't a problem on those bands. The Q-faktor increases and also the receive performance.


Before we start with some construction details, lets look into the calculated patterns and SWR curves.
On 10 meter the vertical is almost full size and provides a 17 degrees take-off angle, current distribution is also shown in pink over the EZNEC 3.0 Model. Impedance transformation 75/50 ohm may be considered.
15 meter is quite similar to 10, Hats are extended with tapered elements, hose clamps allow tuning of the hats for your own prefered mode, CW or SSB.
Another tapered section to prolonge the hats. Tuned to 20 meter provides you with a nice small 20 meter vertical which can be used on a very small place, no need for radials but good ground conditions will bring this antenna to "live". (Pattern for average ground)
From 30 meter we have to insert the air-coils and add a B-matching coil to bring the impedance to 50 ohm.
The final 40 meter vertical dipole, shorted to a handy antenna which you can build easily yourself, suitable to travel and place on holliday locations or just in your QTH garden.


If we compare the patterns to, for example a shorted HORIZONTAL dipole we will be lower in gain but the pattern is omnidirectional and the energy is used where we might need it. Blue is the shorted Horizontal dipole, the black trace is the vertical dipole


Below I propose a building schematic, some room is left for individual tuning to own preferences or to improve mechanical strenght. The antenna described is a testmodel and the prototype has been installed since last summer at the QTH of my antenna fellow Franki, ON5ZO, who has done some contest and DX testing after an initial testdrive by the author.


One of the coils for tuning the antenna on 30Meter or 40 Meter. 11 turns of 4mm² isolated Cu wire will allow you to tune to 7 Mc. When you open the coil, you can bring the resonance to 10.125 Mc.

When adding coils, the impedance drops to about 20 Ohm. A Beta matching circuit is used for impedance-transformaion to 50 Ohm. A balun or coiled feedline will be needed to cancel return currents.

When shortening an antenna this amount, the GAIN will be lower, the bandwith smaller (higher Q),and feedpointimpedance lower. The unmatched feedpointimpedance is 14,91-J26,3 Ohms. The amount of reactance (-j26,3) will serve as capacity in our Beta matching network.


I tuned the antenna at my QTH for 40 meter CW. EZNEC 3.0 is accurate, but building the coils implements some tolerance. Therefore I tuned the antenna with the AEA-CIA HF ANALYZER. Although I could easily tune the antenna at my QTH, We found the antenna off-frequency after transport to the ON5ZO QTH. To be honest, we didn't do much effort to solve this trade off and Franki used the antenna with the autotuner from the TS-850SAT tranceiver.


This project will provide you with a good antenna to work DX on 40 meter on a very limited footprint and even with limited mechanical skills you can build one yourself for an attractive budged.
We, ON5ZO and myself found a little frequency shift in function of the weather. This is a minus in which we will investigate to make it more stable.
It is a nice small antenna to put in the garden and if your partner or neighbour can't stand this type af antenna, your defenitely in the wrong HAM-environment.
The satisfaction is all yours if you work the ultimate DX with your homebrewed antenna-"construction".

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