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Wire dipoles - knots in wire?


#1

I’m going to knock up a single band inverted vee for 20m and I wondered if a knot in the wire near the feed, to act as a stopper for a cable tie would have any negative effect?
It would be very near the feed point.

Also, are egg insulators essential at the dipole ends? I was planning on using nylon cord (like the thin white stuff sold for hanging pictures) to peg out the dipoles, so that would be several feet of insulation surely?

And I intended tying a double fisherman’s knot at the end of the dipole to attach the cord, leaving a tail of wire for tuning. Would that affect performance /tuning?


String insulators in link dipoles?
#2

“I wondered if a knot in the wire near the feed, to act as a stopper for a cable tie would have any negative effect”?

I use knots in that way, and I doubt whether the tiny additional inductance makes any difference.

“Also, are egg insulators essential at the dipole ends”?

No. I also use nylon cord, as I think lots of others do too. It might not be good in prolonged heavy rain, but then a small egg insulator would probably struggle as well.

“And I intended tying a double fisherman’s knot at the end of the dipole to attach the cord, leaving a tail of wire for tuning. Would that affect performance /tuning”?

I don’t think it will affect performance, and the tuning is taken care of by adjusting the tail. Remember that you don’t have to cut the tail, you can make the wire electrically shorter by folding it back on itself.

Just my opinion :smile:
73
Adrian
G4AZS


#3

My linked dipole, about 2 years old, is made with PVC-insulated 7x0.2mm wire (cheap hook-up wire). At each of the insulators I have used tight knots (and knowing my knotting ability probably Granny knots!). About 2 months ago it started a terminal decline. Each and every knot is failing in turn, sometimes between taking it out of the bag and actually putting any tension on it!

For my rebuild (some weekend very soon) I’m going to use the same type of wire. This time I will use some sleeving (probably heat-shrink) to increase all bending radii. I will probably just loop through the insulators and secure with a tie over the then parallel sleeved sections. It the most I might use a simple hitch, but with the sleeving.

End insulators (lack thereof) - I’d worry about damp conditions. I mostly use Richard’s (SOTABeams) laser-cut end insulators rather than a big dog-bone type thing.


#4

I use plastics soffit board for dipole centres. link pieces and end insulators. Porcelain egg insulators are too heavy and are not necessary. Knots will add a small amount of inductance, which would not amount to much on the 20m band. It will mean your antenna is slightly shorter due to the loading effect from the knot. I’ve made 1000s of QSOs without using a balun, despite some people thinking they are essential - even on a simple dipole for temporary portable use

I tie the ends of the dipoles off to a small piece of soffit board, about 3 cm X 2 cm. I drill a small hole in the board, the same diameter of the wire, double back the end of the wire and secure it with shrink sleeving, to which I then clamp a small tywrap for security of the wire to prevent tthe wire end pulling out. If you move up to making a link dipole a similar sized piece of soffit board can be used as an insulator, a double hole can be drilled each side to prevent the wire pulling through and you can then fit a connection of your choice for linking the sections when you need to. Like Adrian suggested, for the ends I then add a decent length of nylon cord so the inverted vee it is well splayed out in use.

73 Phil


#5

thanks for the replies

That’s what I was thinking. Once fully coated in water, is it not the same as damp string.

Good point, and exactly what my mate does. Would that not indicate the electrical length of a know is the physical length?

Interesting. Though 7/0.2 has done well for two years. I have chunkier.

see above?

@G4OBK your reply came in while typing. Some good ideas there. I may have some old uPVC stuff from my window days if I am lucky. I do have a thinnish piece of perspex like stuff (not as fragile) to try with.
I’m not planning a balun, but am planning a choke.


#6

Choke with ferrite = more weight at the tip of a lightweight pole = more chance of a breakage on the pole and on your connections to the choke - in my opinion not essential in practice. Just get the antenna well matched, it is a simple dipole - it will work well so long as it is resonant on the chosen band. No ATU necessary = less to go wrong. Your most difficult decision it what to feed it with, RG58 or the lighter and thinner but more lossy RG-179, which is more prone to an internal fracture of the inner core. I’ve gone off BNC connectors myself, and went on the much maligned yet indestuctible if correctly fitted, PL259 which is now fed into the rear socket of the FT-817 rather than before when I was feeding a BNC into the front. Menu change necessary…

73 Phil


#7

Good points about the weight at the top. I’ll probably (for now) be going fairly sturdily, though tonight I checked the pole and antenna storage in the garage and the lightweight Ali telescopic pole I was going to use next to the car is not there.
I never sell stuff, so that is a surprise and I need to sort an alternative out.

Coax already sorted. Going for the heavier option, but comparable with RG 58, that is RG223. Like 58 but double screened. I have some so a no brainer for now.

Connector wise, bnc not an option for me with my rig, so it’s PL259. I’d use N given the choice, I am very pleased that the vhf output is N. I have a lot of Andrews heliax that will plug straight in, and my antennas for two are N.

My current kit I would definitely not carry up somewhere big like a munro, but will be ok for my local 1 to 3 pointers, even pen y fan really.


#8

PS all. I am loving the quick response here. Joined several Ham boards looking for activity and it’s like calling qrp on 1mm band in a snow storm most places :slight_smile:


#9

I don’t have a problem with knots or lack of insulators for portable operations.
I do however tend to use rubber grommits as end insulators and “chocolate block” as dipole centre’s.

I’m a huge advocate of feeding all my dipoles from the end, which of course keeps the weight low and the feeder down to just a few inches, however funnily enough I’m currently sitting here putting the finishing touches to a centre fed, fan dipole :open_mouth:

I’m already reminded as to why I prefer the easy option of EF…

Anyway here’s my usage of a no knot connection using cable ties, and heat shrink tubing.

Back out to the garden for more trimming…
72
Pete

The only thing wrong with the above statement is the 1mm bit!
QRP and snowstorms do not prevent SOTA activity :smile:


#10

nice and simple! Are you looping the wires attached to coax core over to the right, and the braid to the left, to make the strain relief?


#11

I think Phil means RG-174, not RG-179.

RG-174 is fine for SOTA dipoles the loss is negligible, even towards higher end if HF for a 10m length - doubt you’d need any longer feeder.

A 7m pole is just about ideal for a 40m dipole and up.

Don’t bother with a choke or balun, just more stuff to fail. The only negative due to not using a balun/choke might be an odd radiation pattern, but hey, as long as your signal is going somewhere, who cares? :smile:

I’ve found that RG58 is a bit heavy for the typical SOTA pole and it’s bulky at the rig end if you’re using a small rig like I do.

An inverted vee made with 7/0.2, fed with RG-174 using acrylic insulators will work fine business, I’ve worked a few US stations using only milliwatts and had several QRP SSB VK long path contacts earlier this year with my dipoles.

73, Colin M1BUU

Edit - this was a general reply, not specific for Pete- sri om!


#12

Thanks, that’s very clear. I like it. Do you use choc blocks with wire protectors? I find the standard ones rather chew things up over time.


#13

Actually the braid is to the right (orange heat shrink) and the inner is to the left :smile:
The coax is looped up through the missing centre connector, tie wrapped and heat shrunk underneath.

Looping the coax not only provides strain relief, it helps prevent water ingress as the open end of the braid is pointing downwards.

In a permanent installation, I pot the whole thing in epoxy.

To make it more robust, I’d use heat shrink which passes through the hole in the choc block.

As I said, I don’t generally use centre fed dipoles; this is a bit of a departure for me :grinning:

Pete


#14

Another option for a dipole is to use figure of 8 twin flex. Split enough of it to make the dipole, and put a zip tie to prevent it splitting further. Use the un-split length as the feeder. I used one like that on 30m for quite a few activations, connected directly to the FT817 without a balun.
The advantage is that there are no connections at the top of the pole to fail.
I don’t see any disadvantage in having the balanced to unbalanced transition at the bottom of the feed instead of at the top, as it would be using coax…?

I have lengthened the top span now, and use it as a multiband doublet fed with an Elecraft T1 ATU (still no balun).

73
Adrian


#15

Steve, one of the things that interests me about SOTA is there is lots of scope for playing about with antennas. Not just the antenna principle (vertical, dipole etc.) but also the implementation such as Pete’s chocolate block. Making something that stays put together and doesn’t fall apart with repeated setups and that you can deploy when it’s windy, snowing, cold and you have gloves on is a good part of the challenge.

Anyone can make an antenna. Making one that stays working is a lot, lot harder. Something I’ve achieved with my linked dipole but not with my .64 lambda antenna for 10m. That has needed a repair after every deployment.

Fine on using what is to hand (RG-223). Once the bug bites harder you will be looking at shaving every gram off the weight either to make the bag lighter or so you can carry more stuff! As you are interested in VHF, you may want to consider investing in some of the extra low loss VHF cables. Times LMR or the Japanese 5D-FB are good. Time LRMLite 195 is much lower loss than RG58 and weighs less. That wins on 2 points!

So many things to play with…


#16

Hi All,

One comment to add.

The unun choke has no place at all at the feed point. it should be close to the ATU/Rig. It is put in line to suppress the common mode current (rf on the outside of the coax for example). If you put it at the feed point you are suppressing the 1% max common mode current due to antenna asymmetry. Put it at the bottom and you suppress the maybe 30% common mode current due to feeder alignment etc.

73
Ron
VK3AFW


#17

@G4AZS That sounds nice and simple. is that as an inverted V?

Making antennas has always been my radio interest really. I do enjoy working DX on something I made myself.

interesting. Just checked out their site. Are there decently priced UK vendors?
Does it fit a normal RG58 PL259 OK?
Some of these foil based coaxes have great spec, but can be a 'mare to connectorise.


#18

Funny you should say this.
Its like that in a lot of cases I have found.
A lot of us don;t have a lot of knowledge, let alone rely on others with more experience to help and guide us along. I feel there,s a lot of people out there whom could share there experience of ham radio, so we can further ourselves.

Also If I know something and some one asks about it, only to glad to help and may be even learn a little more myself. Some times, some one can write something down and I just don’t get it, but demonstrated to I learn a lot easier, its just the way I am.

Karl


#19

I guess I’m spoilt in that I am admin on a photography forum, and our response rate is typically minutes if that very often.
Here is pretty good too!!


#20

Yes, usually as an inverted V with the centre supported on my 5m fishing pole.

I do sometimes use a single vertical (or sloping) wire with one or two counterpoise wires for the HF bands, especially if it is very windy. The Elecraft ATU makes it very easy to get an ad hoc arrangement working, though it only handles QRP. More simple manual tuners are available / easy to build, but the Elecraft makes me smile because of its great design features and ease of use :smile:

73
Adrian