Lightweight 40M Dipole?

In reply to G0CQK:

Hi, Jim.

Likewise, I use crimp wherever possible - but with heatshrink to strain relieve the transition out of the crimp. We NEVER use solder here if we can avoid it. The joints can fail in a very short time with the vibration and I’m talking about minutes not years!

73, Richard

In reply to G0AZS:

What do you think?

Dipole is an excellent antenna, but has feed point high above so you need a transmission line/coax between the tranceiver and antenna. Loop can be fed from bottom thus eliminating the coax, but then it is not a self resonating antenna for the purists. Antenna pattern for a low dipole and a loop is about the same. So you can choose: carry the coax/balun or a tuner for matching to a transceiver.

73, Jaakko OH7BF/F5VGL

I have a choke balun on my antenna. I haven’t done any tests to see if it is needed or not. The coil is about 12cms diameter and made from about 5 turns of cable. It’s important that you wind the coil as a soleniod and not lumped. i.e. the turns must not overlap. Keep the coax 180degs apart too. If you don’t then the choke’s performance is badly affected. I wound mine and used some tiewraps and plastic plant tags to keep the turns sequential. I did have a photo on Flickr but deleted it. I’ll dig it out and upload it again to illustrate things.

I reckon my RG-174 feeder has lasted so well because I wind it properly. On the past few activations the cold has made coiling it very hard. So much so I have resorted to just getting it in the bag and recoiling it later at home in the warm. But I try hard to coil it unkinked. Not always easy in a gale!

I think I paid 20p/m for RG-174 at Blackpool last year. I found a 28m reel of RG-316 at another rally for a fiver.

WH Westlake sell both and is a dealer I’ve used many times. There’s a coax loss calculator on his website that might interest a few.

Andy
MM0FMF

In reply to G4ERP:
Absolutely Richard. I think I sometimes over cook it with a double layer of heat shrink.
73 jim

In reply to G0AZS:

No balun on my dipole Marc on the grounds that if the antenna is resonant, 99% should be going out leaving only 1% to come back down. I’ve never had a problem with the 817 which shows full scale on the PO bar chart on all bands that the dipole is cut for. Others may disagree with my approach, but I am no electronics guru and prefer to adopt KISS principles wherever possible.

73, Gerald

To Marc GØAZS.

Hi Marc. There are lots of good suggestions & specs above but for what it’s worth, my dipole details are below. These are built for 80m as the lowest freq. They’re just pasted in so you will have to ignore data that is irrelevant.

Dipole 1 goes back 20 years and but I have had no breakages as such. The two or three problems I have had have been ‘internal’ open circuits at joints in the PVC wire.

Dipole 2 has one less joint and is 24 AWG PTFE throughout. It has served me well except for when I accidentally dropped a heavy rock and cut-through the wire on Fairfield making Jim’s point a valid one.

Mast guys are something I wish to avoid so my mast is 5m long and home-brew (760gm, CFC rod). It is flexible and the dipole is structural in so much that, in conjunction with the 1m end supports, it is ‘expected’ to set the tension and also to pull the mast vertical with the included angle into wind. This it does without complaint except when the two 38gm coils are fitted for 160m band use. It was never designed for this but has ‘coped’ on over 40 summits in windy conditions. This leads me to believe that a good quality multistrand insulated wire of no more than 24 AWG and well strain-relieved, will suit your purpose, especially for a 40m aerial (2x 9.77m). Some TLC is still a requirement however.

Specs:

Dipole 1: Multiband link-dipole (80-60-40-20 with coils added for 160m) made 1988 for WAB; SOTA since 2002, as follows:

Wire spec: White 22 AWG PTFE inboard & 24 AWG grey PVC outboard (wire specs unknown)
Links: Gold-plated pins & sockets from multiway connector, heatshrink covered and strain relieved with nylon cord.
Coax: 8m of RG178 (50 ohm, 1.8 mm dia, PTFE dielectric)
Centre: Solder joints, self-amalgamating tape wrapped, strain relieved & quick-release clip attached to suit mast. (No balun)
Weight including reel made from cable drum from RS 359-475 equipment wire, with PVC pipe centre / handle: 380gm.

Dipole 2: Multiband link-dipole (80-60-40 with coils added for 160m) made 29-12-06, as follows:

Wire spec: (Dipole 2) RS components, 25m reel 24 AWG, PTFE pink. RS stock number 359-475, 500v, 1.2 mm dia, silver-plated copper 7 x 0.2.
Links: Gold-plated pins & sockets from multiway connector, heatshrink covered and strain relieved with nylon cord.
Coax: 8.1m of RG316 (50 ohm, 2.9 mm dia, PTFE dielectric)
Centre: Solder joints, self-amalgamating tape wrapped, strain relieved & quick-release clip attached to suit mast. (No balun)
Weight including reel made from cable drum from RS 359-475 equipment wire, with PVC pipe centre / handle: 360gm.

I wish you much success with your half-Watt rig. I have some 1W rockbound CW rigs (OXO TX, Sudden RX) which I built some time ago but only tried on SOTA once, so I will be interested to see how you get on. I think you will enjoy building a really light aerial to go with it.

73 John G4YSS.

In reply to G0AZS:

From my own measurements a few turns of coax is not sufficent as a choke for the low bands. An alternative is to wind the feeder through a ferrite ring at the dipole feed point to form a current balun. You could use RG174 and an FT114-43 as this would probably be lighter and less lossy than coiling enough turns of coax to form a good choke.

Another good material for dipole centres is Fibreglass pcb board without the copper. Its light, tough and cheap.

Nigel. G6SFP.

In reply to G6SFP:
Thanks Nigel and everyone else for their comprehensive and interesting comments.

As and when the RockMite is built (still waiting for it). I’ll set about trying to make the lightest usable 40m dipole and let you know how I get on with my ultralight activation kit.

73 es tnx Marc GØAZS