by Robin Thompson G3TKF / Clive Maby G4NAQ
The following article describes the construction of what started as a simple dipole for 50/70Mhz. Finally a choke balun is added to stabilise the antenna performance. The project with a few changes, was inspired by Dave G4NKT who got his ideas from an article in Rad Com April 2016.
I used the following:
- 12mm diameter aluminium tubing – 2 x 1.44m and 1 x 1.99m (see below)
- 1 x Moonraker Dipole centre (either a DPC-A or DPC-38 or similar)
- 1 x Sandpiper ½” (12mm) to 1” (25mm) boom saddle
- ½” (12mm) Cable clips Hellerman Tyton SWA46 or similar
- 2 x 3” 3/8 UNF thread bolts with heads cut off (to match thread on dipole centre)
The basic 50Mhz dipole
This is fed via a dipole centre https://www.moonraker.eu/dpc-38-dipole-centre-for-mobile-whips
The centre can be mounted on either a 1” round section or 1” square section boom, because I had the latter that’s what was used. It’s also easier to get a firm mount on a square section tube. Dimensions from the original article suggested 1.44m (57”) per leg for the 6m dipole, but is dependent on the type of centre one uses. There are a multitude of ways to attach the elements to the centre – but I used 3” 3/8UNF bolts, with the heads cut off (Pic 1) they fitted tightly into the ½” ali tubing and with two slots cut on either side of the tube and a ½” Jubilee clip they were a tight fit.
The dipole centre with bolts ready to have the elements fitted.
The 50Mhz dipole was then mounted at a height of 8ft on a “vertical boom” using a rotary washing line socket for support, to check resonance and trimmed to around 50.3Mhz – it was actually flat over most of the band between 50 – 51.5Mhz , I used an MFJ 259 SWR analyser which is perfect for this sort of job . You may have a more modern equivalent. BUT the SWR seemed to shift substantially depending on the position of the coax on the boom – A choke was made (see below) which stabilised things and improved the match dramatically. This is fitted in the final stage.
The 70Mhz parasitic element
I used a ½” to 1” Boom saddle from Sandpiper Ltd https://www.sandpiperaerials.com/product/element-saddle-12-ele-to-1-boom/
Other establishments stock similar parts.
The original article suggests a length of 1.99m with a spacing of 65mm between the 50Mhz and 70Mhz element. The element (NOTE it is all one length – NOT split in the middle!) was added to the boom on a temporary basis with cable ties, by moving the parasitic element slightly either side of the suggested distance from the 50Mhz dipole and many ups and downs of the vertical boom, I found a good match on the feeder where both antennas were less than 1.3:1 certainly close enough for me. My final dimensions are at the end of the article. Yours may well be slightly different.
Spacing for the two elements was supported by two lengths of approximately 100 x 25mm x 2mm plastic, something like an old number plate cut into strips has been suggested. I used ½” cable clips with the spacing (Pic.3 and Pic.4 ) as below to hold it all together.
The Choke
Numerous articles have been written….. Here are 2 attempts from myself G3TKF and Clive G4NAQ built using the same basic steps…
GM3SEK has some interesting details on his website.
I tried a fairly basic one consisting of 5 turns of Aircell 7 or whatever your feeder may be- RG213 may prove difficult to wind! I used a 60mm former. If you choose a can of WD40 or similar it helps if it doesn’t have a lip around the base. I used double sided tape with 4 strips placed at 90 degrees along the length of the can, I then placed a cable tie long enough to go around 5 turns or so of the coax, along each strip of tape. Carefully wind the coil around the can and over the ties, the double sided tape will hold it all in position. When you have wound the choke, carefully tighten the cable ties to hold everything together using a pair of pliers to hold the “lock” while pulling tightly on the free end helps, then slide the choke off the can. Clive G4NAQ has additional pictures of his antenna below.
For ease of fitting the choke to the antenna I made off the PL259 connectors in the shack so it was only a case of waterproofing the connectors before final erection.
Dimensions
G3TKF measurements
- Dipole element lengths 2 x 1410mm
- Parasitic 70Mhz Element 2000mm
- Spacing 70mm (between centres)
To highlight the point of checking with your own mounting method – I asked Dave G4NKT to check his measurements and his are listed:
G4NKT measurements
- Dipole element lengths 2 x 1420mm
- Parasitic 70Mhz Element 1990mm
- Spacing 65mm (between centres)
Only slight differences but worth checking in your own case.
A few final notes: Wear a hard hat when testing any antennas on masts – the results of NOT are in the two pics of me in A&E – not a pretty site!
Other slight changes have been incorporated – maybe an additional article…
I’m looking at reducing the dissimilar metals issue from brass to steel to aluminum – even after just a year in the air, corrosion was very noticeable.
G4NAQ approach Choke
- Step 1 – you need to following tools and an appropriate length of coax at least 1.5m to start with
Step 2 – attach insulation tape around an appropriate tin (the foam cleaner was at hand & that’s the only reason it has been selected!) with the sticky side facing outward (i.e. not to the tin!)
Step 3 – position 4 or more cable ties around the tin (the insulation tape should hold them at this point if you are using anything much larger than RG58) with the tabs at the base end. Add one round of insulation tape at the lid end just to hold them more securely
Step 4 – wind 5-6 turns of coax starting at the base of the tin tightly against the tin… (you may need some assistance at this point).. tie each cable tie into its base securely
Step 5 – cut off the cable tie excesses and slide off from the tin
A completed Balun, just need the appropriate connectors to be added.