Square halos and horizontal-Vs
Part #10 of the "Roger Writes" series - December 2023
I've built a 5-band Cobwebb antenna for the loft. But have found that moving it into the loft has detuned it more than expected, and the tuning is complicated, as you have to balance the loop length, midpoint short, and end gap, for 5 separate bands.
While researching the Cobwebb, I found designs for square halos, these can use the same speaker wire and spreaders as the Cobwebb. The main change is to move the shorting link to the end of the loops. This means that you only have to worry about the loop length, and end gap, so they are easier to tune. In practice, I think there is less interaction between the bands, which helps a lot.
A 40m/20m experiment
I modeled a square halo for 20m & 40m in 4nec2 (with the wires seperated to 1/1000th wavelength), to get the basic dimensions.
I made it using two lengths of cable for the 40m loop (joined at the feedpoint), with a single piece for 20m (with the feedpoint split off of the main cable), overall it's 5.4m (17' 8") square.
This is it about 80cm above ground:
This shows the blue joining wire for the 40m band, and the air-cored choke:
After some tuning, we tried it during one of the contests, it was super quiet, and we worked a few distant stations (and could just about be heard in Cuba and Florida, so about 4500 miles from the Welsh border on 100W).
But, getting a good SWR, and good radiation are two different things. Looking at the model, with it this near to the ground, the radiation is mainly upwards, and the ground is absorbing a lot of the signal.
This is the pattern with the model 1meter off the ground:
Note: The simulated gains given here are caclulated using a "real ground", anything within a 1/2 wave of the ground is massively affected, and will give much less that the 3.6dBi of a dipole in freespace.
If I changed to fibreglass supports, and mounted it on a 7.9meter (26ft) mast, the modelling looks better:
This has a similar pattern, but significantly reduces the ground absorbtion, it shows -1dBi at 60degrees which is better.
When space is limited (like loft mounting) this is about as good as it gets. For field use, raising something as complicated as a multi-band halo on a mast is difficult. So for field day operation, I looked at simpler options.
Multi band inverted-V
A dipole (or doublet) is the simplest, but they needs matching, so I looked at a few inverted-V options. Turning an inverted-V to the horizonal works quite well, with 120...90 degrees between the two wires. A simple horizontal V for 40m & 20m can be arranged as a cross, like this:
For the 40m band the long wires, are around 20m long, so overall this needs about 30m x30m, (100ft x 100ft) so definately an outdoor antenna!
Using a 7.9mtr centre pole, and 6m outer poles, this has a slightly flatter pattern compared to the halo.
This is showing the radiation from the side, with the model indicating -1dBi at 65degrees. The pattern is very omnidirectional.
Adding more bands can be done by adding wire underneath these ones, this is a design for a quad band 15/20/40/80mtr crossed horizontal V.
With this, there is some interaction between the bands. Moving the extra band wires by 45degrees, reduces the interaction. (and they can be supported from the other wires, with insulating nylon cords, so no extra end poles required), this design for a quad band 15/20/40/80mtr star horizontal V is better.
This gives very usable SWR without matching across many bands:
When the weather warms up, I'll go and experiment with some of these designs. The plan is to start with the horizontal V for 20/40m, and then add a set of 80m wires to create a 20/40/80m star horizontal V, then finally add a 15m set, to get the quad band 15/20/40/80mtr star horizontal V.
The Roger Writes series
I research / dabble with lots of things, and figured that if I write my notes here, I can quickly reference them, also, sometimes, they are useful to others!
Here is what I have so far:
This page was lasted updated on Friday, 22-Dec-2023 13:01:11 GMT