I decided to build a multi-band dipole antenna as soon as i found out our new flat had loft access. After previously being constrained to a magnetic loop strung up in front of our living room window this was very exciting. In addition to this i even acquired an extra bedroom in which i could convert into a shack.
Dipoles are relatively easy to construct and are pretty cheap compared to commercially available antennas and in some instances a simple dipole can outperform other ‘compromise antennas’. I estimate that this dipole cost me a grand total of £35 (£20 for the speaker wire itself), not bad going for a multi-band HF antenna! I obtained all the components from eBay.
It’ s been working really well so far (used up to 50w – my current license restriction) and i’m very happy with its performance. I’ve only used it for PSK31 so far but i have had many QSOs on it including JA5BVO (Japan), OX3DB (Greenland), N1HNY (U.S) and PY2TWI (Brazil). Here’s a screenshot of the PSK31 signals that i’ve been putting out:
Multi-band dipole! How does that work?
My initial question about a multi-band dipole was how could this magical thing work? How is the correct dipole length selected for the frequency being transmitted through the coax? It turns out there’s no magic, the correct dipole wire is selected automatically based on the one that offers the least resistance. This is how you can feed all the dipole wires at once with a single coax feed and only have the correct segment radiate the signal.
I wanted to build a simple multi-band dipole that would cover the major HF frequencies that i’m interested in. I quickly had to forget about the 80m and 160m bands due to the length of a half wave dipole (40 meters and 80 meters respectively) and a distinct lack of loft space. I chose the following HF bands:
- 30m (data band)
All of the dipole components were purchased from eBay and included:
- 100m of speaker wire (2 strands)
- Dipole center (you can make your own, but this makes life easier!)
- PL-259 Male Connectors
- PL-259 Female Couplers (for joining two PL-259 connectors)
Speaker wire is formed of two wires, one made from stranded copper and the other one steel. For the antenna i was only interested in the stranded copper wire, so i needed to separate these from each of the lengths i had cut. This is quite easy as the two lengths are insulated separately and can be split easily as a result.
The one problem i had with the speaker wire was that the outer insulation seemed to be greased, requiring me to put the wire on top of a plastic sheet to stop the carpet from being stained. The XYL was out during this procedure and never suspected a thing!
Building the ugly 1:1 choke balun
I also made an ‘ugly balun’ to match the unbalanced coax feedline to the balanced antenna. The purpose of this to help eliminate RF currents from flowing on the outside of the coax, this ultimately ensures that the dipole is radiating the RF and not the feedline.
See my ugly balun construction post for a full step by step guide of its construction.
I used the following components to construct the balun:
- 22 feet of RG8 coax
- Empty 2 litre drinks bottle (to act as a former to wrap the coax around)
- 2x PL-259 connectors
The final ugly balun, it is indeed very ugly (but functional and cheap!);
Constructing the dipole
The following section outlines step by step how i constructed the multi-band dipole.
Preparing the Speaker Wire
First of all i cut the speaker wire in to segments, corresponding to the bands i was interested in, the basic premise being to use a length of wire half as long as the band wavelength you wish to use. Here’s the lengths i used for each band:
- 40m – 20.09 metres of wire
- 30m – 14.12 meters of wire
- 20m – 10.06 meters of wire
- 17m – 7.88 meters of wire
- 15m – 6.72 meters of wire
To calculate the length of wire for other frequencies you can use this formula:
Total length (in meters) = 142.65/ƒ
For example if we wanted to add the 12m band (24.940MHz), we’d do the following:
Total length (in meters) = 142.65/24.940
Total length (in meters) = 5.72 meters (rounded up to two decimal places)
Once you have your length of wire for your desired band you will need to cut the wire in half again for (one for each side of the dipole).
You can of course trim the lengths at a later date to get the perfect SWR, but i found the lengths above to be about right.
Soldering the dipole center
At this point we have a whole bunch of wires and a dipole center. I found five wires was about the maximum i could thread onto each center tag (ideally i wanted to add 6m to the antenna but had to forget this idea). The next step was to solder the wires to their corresponding solder tag, i found it easier to first thread each wire through and then wrap the wire around the end tag, once i had done this for all five wires i soldered the whole lot in one go:
I then repeated this for the other side of the dipole. I wanted to keep things neat so i also secured cable ties around each side. Here’s what i ended up with:
We now have the completed multi-band dipole, so lets get it strung up…
Mounting the Dipole
I mounted the antenna in my loft, so i didn’t need to worry about weatherproofing it. You want to mount the center feed point as high up as possible in the apex of the loft to keep it away from electrical wiring and copper boiler tanks!
You also want to make sure that each wire comes off quite separately from the others at the feed point:
I used a staple gun to support the wires throughout the loft.
Let the final 4 inches of of the dipole wires hang free away from the wood. This is for reasons of safety and stops the dipole arcing to the woodwork:
I found that my loft wasn’t big enough to have the 40m and 30m wires out in a straight line, to get around this i zig-zagged them along the loft:
The End Result
The final result of my construction of this antenna is that it outperforms my other expensive commercially available antennas! The end product is cheap and i’ve had hundreds of QSO’s on it so far (using up to 50w) including some nice DX. The SWR always seems to be very low across the bands and the reflected power rarely exceeds 3-4 watts (assuming 50w from transceiver). All in all i’m very happy with it and there is a massive sense of achievement when you make contacts from an antenna you made yourself.
This marks the start of my interest in antenna construction and i’m sure this won’t be me last one.