I acquired this TFD-BBE model (terminated folded dipole) antenna from South Midlands Communications, Eastleigh a few years ago and finally decided it was time to put it up as I was missing having an antenna that would cover the 80m & 40m bands.
This TFD is center fed and offers coverage from 2.5 to 30MHz with a maximum power of 250W and generally good VSWR across the bands. It doesn’t need a tuner, although I think having a tuner in the circuit is a good idea. Its horizontal length is 28.5 meters which was a bit of an issue for me having a garden only 30m in length - to counter that I decided to go for an inverted vee configuration reducing the horizontal length to a more manageable 24m and offering approximately a 30-degree angle slope. Finding a vertical support solution took a bit of time.
The trigonometry gave me a minimum height of 8m to achieve the above. I started with a 6m steel scaffold pole only to realise the weight of it proved a challenge, not only to handle but also to erect, and being conductive I figured this wasn’t a good idea with a bare wire antenna. I looked at various push-up sectional fibreglass masts readily available at Ham radio suppliers, however with an un-guyed head-weight (I don’t have room for guy lines) of a bit over 5kgs they advised against it as it would be exceeding the head-weight limits for most of those I considered. By chance I came across these GRP (glass re-enforced plastic) 50mm OD and 38mm OD tube handrails which seem to be replacing steel tube railings. These GRP handrails from BMSteel in Glasgow, are 5mm thick and are pretty ridged and importantly, only weigh around 6.5kgs for a 6m length. Also, and most conveniently, the 38mm rails fit snuggly into the 50mm tunes with less than 1mm gap, this way I was able to purchase 1 of each, cut down the 38mm tube to 3m to give an 8m mast with 1m of the 38mm tube inserted for support. I secured the 38mm to the 50mm using a threaded bolt and nuts through them both. I probably could have gone higher but as I’m living in a very windy location, I figured best to stick with the 8m. The GRP handrails come in a nice yellow, some might like that, but I’ll be spraying them matt black when the weather permits, the paint will also reduce UV deterioration of the GRP.
Support for the mast
Although there are many ways to erect a mast, I decided to use the tried and tested method of suspending the mast between two 100 x 100mm treated 2.4m fence posts spread 90mm apart, and to have an 8mm threaded stainless steel rod supporting the bottom of the mast to enable a controlled tilt-over. I inserted the rod in thick polythene tubing to prevent metal on GRP contact.
As I have an overhead power line at 10m height to my house I was careful to ensure (I actually measured it) that any failure of the mast and antenna would not fall upon the overhead power cable; as it happens, I have double the height of mast clearance so there is no risk – it’s obviously essential that we always follow the safety guidance we learned when doing our licenses. Further up the posts I used a 51mm hole saw in 90mm x 40mm (2x4’s in old money) timber for support at the top and midway on the posts.
Ideally, you want to sink your fence posts a third underground, but as I sit on bedrock at around 600mm deep I had to have a larger foundation for the posts, we dug the posts in approximately 450 x 450 x 600 mm hole which took 5 x 20kgs bags of Postcrete fixing cement. A lesson learned is that you will need more Postcrete than you think, I used two more bags than I thought I needed and could have used another one, but we made do. I sprayed the bottom three feet of the posts and the top ends with underseal bitumen hopefully to delay any rot, apparently wooden posts tend to rot at ground level first. The whole lot will get another coat of wood preservative when the weather improves.
The Antenna Support
The TFD is usually hung offset from the mast on a bracket of around 40cm. However, not wishing to alter the centre of gravity too much, I used a threaded rod offset by about 10cm on which I hung a small pulley to hoist it up with.
The mast being GRP I was less worried about the bare wire elements connecting with the mast than I would have been had it been a metal structure. I’ve used 2mm Kevlar cord with a 200kgs breaking strain to hold the antenna up the top. I might choose a thicker cord next time as 2mm is a bit thin to work with even though it is certainly strong enough for the job. I did install a guy line support at the junction between the 50mm and 38m section. Even though I don’t really have room to have guy lines, I figured I might be able to find a way to add support if windy condition proves too challenging for the mast on its own. I wouldn’t achieve the recommended a 45-degree guy line angle, but less might be possible so I put it there for good measure. Of course, the antenna is easily dropped to a lower level when needed.
Erecting the Mast and Mounting the Antenna
The antenna’s assembly was straight forward, however be careful if you have to uncoil thicker non-flexible wire it can easily get out of control and you’ll end up in a fankle. Of course I was careful, sort of. Give yourself plenty of room and uncoil your antenna as close as you can along the axis. I used ABR RG8X 50 ohm coaxial cable for run of approximate 16m to the shack, primarily because its light weight and will easily handle the 250W limit of the antenna. I hoisted the centre feed terminator and balun (Unun) and then stretched the opposite sides of the antenna out to a 30-degree angle, ensuring both end insulators were at equal height (2m) and that there was equal tension on either side of the mast.
VSWR Checks and First Air
With the antenna securely up and the coax cable run to the Alpha Delta surge protector grounding panel, then into the shack, I was able to connect the antenna analyser and check the VSWR across the 2-30 MHz range of the TFD. I was pleased to find that it very closely matched the manufactures VSWR plot supplied with the antenna. Nothing above 2.2:1 and generally less that 1.5:1 and a flat 1.1:1 across the 20m band. Although probably not essential, the antenna is easily matched using a tuner.
The TFD isn’t an antenna that will offer you global DX, but I have been able make good contacts in near Europe and as far afield as Bulgaria, Italy and Spain. Perhaps I’ll be able to push it out further in due course. It also has a high angle of radiation so it has NVIS characteristics which is handy for short range UK contacts. It's also convenient that it covers all frequencies between 2-30MHz – in particular having the ability to join in on the UK 60m Nets (5.379 MHz USB) on this little used band.
In Summary
There is great satisfaction in completing a project that gets you on the air. For novice operators, I would always suggest you call on the experience of others, that can save you a great deal of expense in both cost, time and effort. We are fortunate to have years of experience in our club and if you can’t find the help you need there, YouTube is a fabulous source of information on everything amateur radio.
In closing this blog - I need to once again advise caution when erecting a mast, antenna and/or working at heights. It can be dangerous - always plan carefully and assess the risks before proceeding.