Better horizontally than vertically polarized
If you only have limited space for an antenna, but still want to DX on HF on multiple bands, you will soon have to rely on a vertical antenna. DX’ers among us know that on the 10 and 20 meter band you can often work better horizontally than vertically polarized. At a half-wavelength height, a horizontal half-wave dipole already performs better at low take off angle DX, than a quarter wave vertical. But a dipole at 20 meters is more than 10 meters wide. That does not fit every home or garden. Yet there is a horizontally polarized antenna for 10 and 20 meter band, which is suitable for a limited space.
Halo antenna as inspiration
Shortened multiband dipoles for 10 and 20 meter bands are widely commercially available. But with 7m to 8m wide, they’re still large for a small roof or garden. In addition, shortening elements generally also means giving in on performance. Because a dipole is directional, you also need a decent rotor. At only 2.8 x 2.8m, the alternative takes up little space, doesn’t underperform in comparison to a full width dipole and is also omnidirectional. It’s the dual band Square Halo. The inspiration comes from the Halo antenna, a popular horizontally polarized omnidirectional radiator for VHF and the Cobwebb antenna, a horizontally polarized omnidirectional radiator for HF.
The folded dipole as the base element
If you have taken an amateur radio exam, then you have undoubtedly learned about the properties of a folded dipole. When stretched out, such a dipole for the 20 meter band is approximately 10.5m wide and has an impedance of 300 ohms. That folded dipole is the basis of the Square Halo. If you put it a circular shape, with the ends just not touching each other, you get the famous Halo antenna. Still in use here and there for reception in the 3 meter band. The nice thing about this round shape is that the folded dipole then has an impedance close to 50 ohms. In addition, the Halo is a near omnidirectional antenna. You don’t need an expensive rotor!
Make it a square halo
It’s constructively quite a challenge to make a 10.5 m long folded dipole, like for 20 meter band, in a circular shape. But making it circular is not necessary. When you bring the dipole into a square shape instead of a circle, it has almost the same properties. In America, this antenna shape is also known as the Squalo, amalgamation of Square and Halo. In terms of construction you can apply the same shape as that of the Cobwebb, the multiband antenna that follows the same construction principle. The antenna is then only 2.8 x 2.8 m wide. You can then apply the same principle for the 10 meter band and place it within the square shape of the 20 meter dipole. Both dipoles can be fed with a single 50 ohm coax cable, without any matching network.
Not a Cobwebb antenna
From an electrical point of view, the Square Halo is not similar to the Cobwebb antenna. The Cobwebb, designed by G3TPW Steve Webb, uses a type of folded dipoles with open ends. By shorting the radiators half way through, a kind of T-match is created to bring the impedance to 50 ohms. The disadvantage of this design is that the elements are very difficult to tune because in addition to the total length, you also have to move the short-circuit point back and forth. The Cobweb, with one b, is an ‘easier’ to tune and build version, designed by G3TXQ Steve Hunt (sk). It uses normal half wave dipoles and a 4:1 balun to bring the impedance from approx. 12 ohm up to 50 ohm. Our Square Halo uses half wave folded dipoles which do not need any matching at all. They can be directly fed with 50 ohm coax.
How it performs
When I built the antenna myself and deployed for the first time, it was mounted only 7m above the ground. At that height, the antenna on the 20 meter band performed just as well in terms of transmission as my 13m long full-size 5/8 vertical. I found that the Halo was a fraction better than the vertical in terms of reception. Later I raised the Halo to 12m. At that height it outperformed the vertical by at least one to two S-points. The bandwidth of the Halo is a little better than the Cobweb(b). On the 20 meter band, the SWR over 250 kHz remains below 2:1. At 10 meters I measured the bandwidth over 400 kHz.
20 meter band performance simulation
I did a MMANA simulation of the Square Halo on 20 meter band at 10m height. The gain is 0.5 dBi. That is just 1.1 dB less than a regular half wave dipole… when you compare it to the side of the half wave dipole where it radiates at max, perpendicular to the radiator. In direction parallel to the half wave dipole’s radiator, the Square Halo is about 6 dB better. Simply due to the fact that it is a near omnidirectional radiator. Again, you don;t need a rotor.
Building it yourself is actually quite easy
The Square Halo antenna for 10 and 20 meters is not available in any store. You have the option to purchase a Cobwebb, levae out 12, 15 and 17 meter band and adjust the radiators to the Halo principle. But building (partly) yourself is actually quite easy, costs half of a new Cobwebb and is also fun to do.
Folded dipole from plain speaker cable
You can easily make a folded dipole from ‘figure eight wire’ speaker cable. Like 2x 0.75mm2 is already fine if you work with 100 watts. But 1.0mm2 or 1.5mm2 is also good. But be ware of the cable quality. A lot of cheap cable is CCA, which is aluminum with a very, very thin copper layer and like impossible to solder. I recommend buying 100% copper (CU). You need a length of 5.3m and 10.6m of cable respectively for the 10 and 20 meter band. Right in the middle, cut one of the two wires and cut back about 2cm from each strand. Place a ring connector with a 5mm hole on each side of the cut wire. Solder the connector to the cable instead of crimping it. The connector can come loose from the cable if there is some tension. Not funny if your antenna is already high in the mast! If you remove the plastic sleeve from the connector, you can easily solder it to the cable.
At the ends of the radiator, strip both wires one centimeter and solder them together Then fold them back about 2cm and tighten the cable with a cable tie so that you get a kind of flat loop. By varying the folded back length, you can later adjust the radiator. A disadvantage of multi-core cable is that moisture seeps through the entire cable due to capillary action, resulting in copper corroding. You can prevent this by finishing the ends of the cable with nail polish (the thicker gelly type). You can then choose a fancy color or one with glitters! I use the XYL’s real stuff: 60 Seconds Super Shine from Rimmel London.
Building the common mode choke
A common mode choke does exactly what it says. It strongly reduces high frequency currents flowing over the coax shield. A common mode choke is a necessity with our antenna if your want to prevent RFI. You place it at the end of your coaxial cable, as close to the antenna’s feed point as possible. You can buy one of make one yourself.
The air core coax version
RG-213: Roll up 6 times over a diameter of approximately 15cm.
RG-58: Roll up 8 times over a diameter of about 10cm.
The feritte core coax version
Coax 6 turns of RG-58 over one side of an FT240-43 toroidal core. Cross straight over underneath and make another 6 turns on the other side of the toroid core.
Where the air core version is mainly effective at 20 meters, the ferrite core version also provides considerably better damping on the 10 meter band. Both versions can handle up to about 400 watt in SSB.
The feed point
You can easily connect a dipole center as the feed point to your air core choke. You can find these dipole center connectors at (online) radio amateur shops.
For the ferrite core version, solder 5mm hole ring terminals first to the choke’s coax center and shield. Put the ferrite core choke in a splash-proof plastic case or box, 10x10x5cm should be big enough. In the box you mount an SO-239 chassis part to which you solder the other end of coax. If you drill two holes of 5mm at a distance of 4cm from each other on the bottom of the housing on the opposite side, you can insert two stainless steel M5 screws through the holes and ring terminals. Wing nuts are very useful for attaching the radiators to the screws at the bottom of the plastic feed point box.
The mechanical part
The Square Halo is held in place by four cross-shaped spreaders. There are many solutions to be found for mounting to a mast. You can choose to make the aluminum fastener yourself. However, after comparison I came to the conclusion that the costs are only a fraction lower than a ready-to-use kit. G4ZTR John Lemay sells such a complete kit in his webshop Aerial-Parts of Colchester (aerial-parts.co.uk) for GBP 38.00 (price in February 2021). Including shipping to The Netherlands I paid EUR 62.00.
The G4ZTR kit uses aluminum tubing as holders for the spreaders. The spreaders themselves are not included in this set. These spreaders must protrude at least 190cm from the center of the antenna and must not be made of conductive material. Fiberglass tubes work great. Those with a diameter of 22mm fit perfectly in the aluminum spreader tubes of the Cobweb hardware kit. I bought them (type number ‘RF22’) in 150cm lengths from a kite building parts shop. But you can also use fiberglass fishing rods.
By drilling a 2.5mm hole in the aluminum holder tubes you can secure the fiberglass poles with a 3.5mm parker or self-drilling screw.
The feed point should be mounted approximately 100cm from the point where the spreaders intersect. The aluminum tube with which the antenna is attached to the mast is made for this. You can attach the dipole center or box containing the common mode choke at the very end of that tube with two sturdy cable ties.
Attaching the folded dipoles
First place a mark on the spreaders at 190cm and 100cm from the center of the spreaders. Attach both dipoles to the feed point first. Then secure them with a cable tie to the spreaders at the location of the markings.
Then bring the ends together so that there is about 10cm of space between them. I myself use textile elastic to keep the ends together.
Tuning the dipoles to your desired frequency
Before tuning, place the Square Halo high enough so that the ground and surrounding objects have as little influence as possible. Start with the 20 meter dipole. Once it’s tuned in, you do 10 meters. The 10 meter dipole has less influence on the 20 meter dipole in terms of tuning than the other way around. It is best to use an analyzer or else a SWR meter. You can tune each element by making the ends longer (lower the frequency) or shorter (increase the frequency). You don’t have to cut anything, just change the length of the folded back cable ends. Then secure the dipoles to the spreader tubes with a second cable tie. You don’t want the elements to slide across the spreader tubes with a littel wind!
Expand from two to five bands
The special thing about this antenna is that it is easy to expand to more bands. That is especially useful if you are restricted to certain HF bands due to your license. Once your upgrade your licence, you can add 12, 15 and 17 meter band dipoles. I have expanded my Square Halo with these three additonal bands. The tuning takes the utmost of your patience, but once it is done, you have an easy to use five band antenna.
My part list for the Square Halo web
- 16 meter speaker cable (100% copper).
- 4 pieces 22mm fiberglass tube 150cm long.
- 4 self-tapping screws or self-drilling screws 3.5×9.5mm.
- Bag of cable ties at least 10cm long.
- 4 ring connectors 1.0-1.5mm2 5mm hole diameter.
- Elastic band.
- Optional: Cobwebb hardware kit