Other SOTA sites: SOTAwatch | SOTA Home | Database | Video | Photos | Shop | Mapping | FAQs | Facebook | Contact SOTA

Feeding a resonant dipole with ladder line

Some technical advice on feeding a resonant mono-band dipole with ladder line please folks:-

I’m trying to mount a dipole on some reasonably high masts (somewhere between 10 & 20 meters above the ground).

Normally I would use 3 supports (one for the balun in the middle & one at each end) but in this case I only have two supports available. I think that the weight of the balun & feeder cable in the middle will cause the antenna to sag by an unacceptable amount.

Yes, I know that most people would normally feed with ladder line & use a balanced tuner (I guess that this arrangement then effectively becomes a doublet).

For a couple or reasons, I don’t want to mess about with tuners and wondered if I could feed it with ladder line but not bother with the tuner.

This antenna is going up at a remote site. The antenna will be set up permanently but all of the radio equipment will be removed when not in use.

The site will be used by several different people at different times who will be bringing their own radios, but using the fixed antennas. Some of them don’t have a suitable balanced antenna tuner. I also want to minimise the amount of complexity with tuners & the amount of equipment needed.

Basically I want to have a plug & play resonant antenna that I can just plug the coax straight into any radio without the requirement for a tuner.

Can I feed a resonant dipole with 450ohm or 300 ohm ladder line down to the ground and then put the balun on the ground (so that it isn’t hanging on the entenna wire)?

Google posts seem to be a little vague but what I’ve seen so far seems to suggest that I need to use a ½ wave of ladder line (factoring in the velocity factor of the ladder line) to get the same impedance/SWR at each end of the ladder line.

If I have this correct, I should be able to use ladder line to connect to a 1:1 balun at ground level, then connect coax running directly to the radio without the requirement for a tuner.

Have I understood this correctly?

The ZS6BKW antenna uses a fied length of ladder line which then feeds a co-ax and with some careful pruning loads up on several bands. https://www.nc4fb.org/wordpress/zs6bkw-multi-band-antenna/ . Although this isn’t really the question that you asked it works in the way you need - and it semms to perform reasonably well. 73

Paul

Yes, this will work with a 4:1 balun, in effect it is a comudipole antenna as first described by PA2ABV. Another approach is to replace the ladder line with the twin wire sold for speakers, this has a much lower impedance than 300 ohm window ribbon (I seem to remember that it is about 75 ohms) and will give an acceptable SWR.

Just use coax. You don’t need a balun to connect coax to a dipole.

This whole balanced/unbalanced idea causes so much confusion. There are differential mode currents and common mode currents. Coax carries both and ladder line carries both.

If you want to reduce noise, a lightweight current balun/choke is a good idea, but not necessary. If you want to use one, I recommend the QRP balun/choke from Balun Designs. It only handles 250 W, thus “QRP”. Get the additional mount they offer.

For any coax, use a strain relief. That can just be a loop of coax tied to a support hanging down from the feedpoint. You don’t want the weight of the cable supported by the connector.

wunder

1 Like

Hi James,
You need to consider the dipole and the transmission line (you are proposing 300/450 ohm ladder line) separately.

The dipole at its feed point at resonance will have an impedance of around 72 ohm if horizontal (not inverted vee). The impedance depends on the height of the dipole above ground in wavelengths.

The transmission line will act as a transformer depending on the length of the line in wavelengths. So if the length of the ladder line is half wavelength (or multiples of this) it will be 1:1 and so you will end up with circa 72 ohms at the end of the transmission line. You could then place a 1:1 current balun here and feed with 50 ohm coax with a resultant SWR of less than 1.5:1. However, if the transmission line is quarter wavelength (or multiples thereof) then you will end up with a much higher impedance.

So it depends on which band your are aiming your mono-band dipole for. If you went for 40M, which is commonly used for SOTA, and placed the dipole as near to 20M AGL and fed with 20M of ladderline with a 1:1 current balun at the base I think it would work extremely well. If the 40M dipole is at 15M AGL then still feed with 20M ladderline but you would need to deal with the circa 5M extra. Just keep it off the ground.

Paul’s suggestion of the ZS6BKW antenna is a good one and I think you should consider it. The length of the ladderline is critical and in practice is about 11.9M but would need to be tuned.

Hope this helps.

73 Andrew G4AFI

Hi Andrew,

You are correct in your conclusion that using an electrical half wave length of 300 ohm line would give you the same impedance that you have at the antenna feed connection. The line will have an SWR of the order of 6:1 but it will be hard to measure the resulting loss as it will be low.

The ribbon will obviously be lighter than RG213 or RG8 type coaxes but not RG174 which will be fine for runs up to 20 m on HF.

You can put your 1:1 balun at the bottom of the feeder and choke off any residual common mode currents.

A 4:1 balun would be incorrect.

The feed resistance is likely to be nearer 50 ohms than 72 ohms although it does depend very much on the height above ground.

Most people have a support in the middle and use coax bound to the support at the top and at points on the way down. The preferred arrangement is an inverted vee as this has a more uniform azimuth pattern and a better elevation pattern than a plain dipole.

The problem you will face with your planned arrangement is fatigue fracture of the unsupported feed line. It will move in the breeze and eventually break especially if it is about 16 m long. Mine broke after 2 years. You will have the same problem with thin coax.

You will still need to start with your dipole longer than you calculate is the right length and trim it for the desired resonance. Add 5% for the inverted vee arrangement.

Happy chasing.

73
Ron
VK3AFW

3 Likes

I am no antenna expert but can see that some of the suggestions to not fit the original question.

This does not refer to an inverted V but to a two pole support which implies a shallow V of depth depending on wire tension and feeder weight. Any strain relief for the feeder would still strain the dipole centre fitting and the dipole wire attachments to this fitting and suggests to me a maintenance and reliability problem unless a fairly deep V was acceptable.

BTW - does anyone know why I have an invisible cursor here and so keep losing my place?

73,
Rod

Thanks for the replies folks. It’s pretty much what I thought.

Just to reply to a couple of comments:-

This is one possibility that I was looking at, though I’m swaying towards a dipole because it is an antenna that I am more familiar with.

We want to run this setup at full legal power (400 watts in the uk). I’m also a great believer in over-engineering things (especially if this is going to be used by several people from a local radio club), so I want to over-rate the components for around 800-1000 watts.

I know that we shouldn’t be running that much power in the UK (personally I don’t run more than 100 watts, and I’ve genuinely never gone above 300 watts) but there are a few people that do flout the rules. I therefore want to make sure that anything that I set up for the group to use is going to be over-rated & capable of taking a bit of abuse!

Exactly what I wanted to know. I thought that this was the case but I wasn’t sure. Hence why I wanted to ask the question before I put any time, effort or money into building it!

Quite correct!

I plan to use fairly heavy duty fence wire (which is designed to be kept under quite heavy tension), so the antenna elements themselves should hold up will.
I still need to figure out the centre arrangement, which will obviously need to be a fairly robust insulator for this setup.

For the masts at each end, I plan to use heavy duty steel scaffold poles (48mm diameter with a wall thickness of 3 mm), guyed in three directions with the stakes concreted into the ground.Probably going to end up using steel guy ropes.

We should therefore be able to put a bit of tension on the dipole to minimize the amount of sag.

As people have said, the weakness is going to be the feed point. That said, if it lasts around 2 years before I have to pull it down & renew the ladder line & insulators, I will be very happy with that.

We plan to do this for 20m, the top of the masts will be around 10 meters above the ground, so we are looking at around approximately 1/2 wavelength above the ground.

Thanks for the advice everyone.

I’m very glad that you ask this question. I’m also experiencing the same problem over the last few weeks. The cursor dissappeared one day after whoever knows what action or upgrade in Discourse software or whatever and it never comes back.
It’s a big disadvantage when writing any posts here.
I would be very grateful if someone can do something to get the cursor back visible again.
73,

Guru

No problems with the cursor here.

I wouldn’t and I’d rather use non-conductive guy ropes, like the ones I currently use for my tower. I use 2 groups of 4 guy ropes each of 6mm diameter Polyester cord (550daN + 5% resistence) at 3 and at 6m of height from the tower base. At 7.5m of height from the tower base I have 4 guy ropes each of 4mm diameter Polypropilene cord (280 daN resitence). Since I’ve had these sets of guy ropes for several years under extreme climate conditions like a hammering sunshine, sub-zero temperatures and lots of water, snow and ice, and they have never broken, for security, I added last summer at the top a new set of 4 guy ropes of 3mm diameter of Dyneema SK75 cord which has an incredible amazing resistance to elements and stretching forces.
imagen
73,

Guru

1 Like

Thanks Guru.

I had considered that but wondered how well it would stand up to prolonged high tension.

Upon reflection I think you’re right.

As you say it is quite strong, should be ok with the conditions here in the UK, and probably a lot cheaper too.

And the most important: you get rid of any coupling or parasitic effects with the steel guy ropes potentially creating common mode currents, perturbing your reception and affecting your antenna ratiation lobes and pattern.
I bought and installed my non-conductive guy ropes back in 2006, when I was living in Aranjuez-Madrid, where the temperatures in summer are well over 40º C for several weeks a year and well below zero several nights in winter time.
Then I dismantled everything in 2012 and reinstalled it in my current QTH in Navarra in 2013. Not a problem over 12 years in service (+1 year in storage when we moved house) securing a 7.5m tower with a 3m mast at the top, plus a rotator and a 5 elements tribander yagi for 10, 15, 20m and 3 different dipoles hanging from a perpendicular steel arm near the top and a 2m band vertical antenna at the very top end of the mast at he top of the tower. The weather in Navarra is not as hot as in Madrid but we also do get some few days over 40ºC every summer and also lots of rain, some snow, ice and pretty heavy winds.
I think you can definitely trust these sort of ropes. They are designed for use in boats in marine environment, which is probably the most adverse in the world.
73,

Guru

As I said, the balun/choke is optional. It would reject locally-generated noise. If this setup is rural, the nearby noise should already be low.

I recommend antenna wire, not fence wire. Use 12 AWG copper-clad steel (Copperweld). That is strong, durable, and the copper layer makes it efficient. This is a US supplier for reference.

Hy-Power Antennas makes a strong center connector for coax. I don’t see it listed separately on their page, but it is a one-man operation, so contact them and ask. Or you can have them make the dipole for you and save some hassle. Excellent products. I’ll follow up with a photo of the center connector and how the wire is attached.

wunder

2 Likes

Why not just end feed it at the top of either support via a suitable matching network or transformer. Then just run the coax down the support to your transceiver. 1/2 wave end or centre fed the radiation pattern will be the same and you’ve eliminated your feeder problem. Probably as about plug n play as you’re going to get.
Paul G0ILO

1 Like

Why do I always feel stupid when someone points out a blindingly obvious solution which I’ve completely overlooked!!!

I only see one potential problem with it (which I’m probably completely wrong about):-

Most of the advice I’ve read about EFHW antennas seems to suggest that the transformer needs to be close to the ground with a ground rod.

I don’t know how true/critical this is. I can always connect the earth to the mast (which will be well grounded for lightning protection), effectively using the mast as a counterpoise. I suspect that this might cause some unintentional radiation from the mast and screw up the radiation pattern though?

Thoughts?

No it doesn’t need to be ground mounted but a very short counterpoise, 0.05 of a wavelength will suffice or just ground to your vertical support. I think we all get an antenna in the air and have a vision of perfect symmetry with regards its radiation pattern , but unfortunately that’s rarely the case. If only we had RF glasses lol. I bet if you could mount both antennas in the same location and switch between them you wouldn’t be able to notice any difference. Understanding what you want it’s the route I’d take.

Paul G0ILO

1 Like

Thanks Paul. It sounds like a very promising candidate. Just ordered the parts for the matching unit.

The whole thing is going to be mounted 10-15 meters in the air on steel scaffold poles. As you said, I might as well use the mast as the counterpoise.

Some interesting stuff here. Same applies if you use a transformer of course . Good luck with it. http://www.aa5tb.com/efha.html

Using Firefox?