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Using 2 dipoles to make a Yagi?

Equipment Antennas
#1

My attempts over the weekend to reach VK/ZL on 20m SSB were ultimately unsuccessful although my call was twice faintly heard but not strongly enough to complete a QSO.

I was using a dipole on a 6m mast which I thought would give the best performance with my 10W transmitted from a low hill. Of course finding a higher summit would probably have been better but was there a way I missed to improve the antenna?

I have two 6m poles with me and two dipoles, a 40/30/20 and a 20/17/15. The overlap at 20m is an accident but can I use what I have to make a Yagi? A search online suggests putting the poles 2.5m apart would give about the correct spacing. For 14.300MHz the spacing should be 2.56m but I doubt that accuracy can be achieved, especially if there is any wind blowing.

Stringing the two dipoles so the angle of the V is the same for each should create a Yagi type antenna with the feed going to the front dipole of course.

I seem to recall someone on here has done something similar with Delta loops but has anyone any experience doing it the way I am suggesting? If I can find a suitable opportunity I will give it a try but any feedback would be very helpful. For example, what impact does the Reflector dipole have on the SWR of the Radiator? Does the Radiator need to be shortened. In a purpose made Yagi the Radiator is shorter than the Director but if I can gain a dB or two it can only help.

73
John

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Directional wire antennae
#2

I remember seeing some notes about doing something similar with verticals, but I don’t think 6 metre poles would be quite high enough to implement that on 20 metres.

Of course, for a yagi, a reflector needs to be slightly longer than the driven element, and a director slightly shorter.

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#3

Hallo John

Did you read the thread of Ignacio @EA2BD? Maybe it will help you…

Even a longer wire as a reflector can make a difference.

73 Armin

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#4

Not if you want the impedance to stay the same. And no, the directors are shorter than the driven element(radiator) and the reflector is longer. But as soon as you start putting all those bits of wire in close proximity everything starts affecting everything else.

Here’s a hint. Make a 1/4wave GP. It will on average outperform an an average inv-v dipole. On average being key. People did well with inv-v dipoles, but the height above ground, ground composition and ground slope can dramatically affect whether the radiation pattern from your antenna is a DX magnet or cloud warmer.

And the other hint: operate from a cliff edge or good slope facing the direction of the DX (not always possible though).

7 Likes
#5

Armin,

Thank you, I had forgotten about that.

#6

I made a 1/4 wave GP for 6m last year but hadn’t thought of making one for 20m Thank you, I will look at that when we get back home.

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#7

I think current has to flow in the reflector (and director) so as well as getting the lengths and spacing somewhere near, you’d probably want to remove the feeder from the passive dipole and connect the two legs together.
It might be possible and more effective to feed both dipoles, but getting the phasing right would be fun - taking account of spacing and electrical feeder lengths…

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#8

Hi John,
if you did manage to make a directional antenna by changing one of the (I presumed linked) 20m dipoles into a reflector (changing it into a director would need it to be shortened - changing it to be a director “might” be possible by adding a loop of wire in place of the coax on the (Balun-less) feedpoint T-piece). - How would you turn the antenna?

As others have said, there will be interactions and so this is a complex undertaking.

I have tried a 20m version of the HB9SL VP2E antenna on a 5m mast but I have no conclusive facts as to whether it is better than an inverted-v dipole - it’s certainly larger and heavier. I also have vertical Moxon antennas for 15 & 10m that I still need to test properly but these are also not something you would like to have to carry and set-up on a small steep summit but would be OK in a park or on a flat clear summit.

My suggestion is to invest your time in improving what you have. Can you get a taller mast? Can you fit better (i.e. lower loss) coax and are you using speech compression? This last one is very effective if you are hearing the VK stations OK and they can’t hear you turning on good speech compression can give an effective improvement of up to one S-Point in readability.

Welcome to the never-ending search for that “better antenna”, enjoy the fun findings along the way.

Oh yes, and if there’s a forest on the summit don’t try vertical antennas and get as far away from the trees as you can with the horizontally polarised antenna.

73 Ed.

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#9

John,
In addition to what Andy said if you are on a flat summit you could raise the centre from 6 m to 9 m and raise the ends by 3 m then at a 10 degree takeoff angle your signal will be 3.5 dB stronger.

Of course the standard 45 W amplifier will lift a 5 W signal by 9.5 dB which would be my approach if I was copying the dx.

73
Ron
VK3AFW

3 Likes
#10

I had missed that point! If I do try it a simple length of wire would be much easier than trying to adapt a dipole.

That is probably a lot easier to achieve than my original suggestion.

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#11

I have used wire yagis for years. They do work.

I ran a QRP station for Field Day one year with a wire yagi on 15 meters and made over 350 contacts with 10 watts.

The hardest part for SOTA is setting up a wire yagi on a summit…you need some space and two or more verticle poles. Hard to do on steep summits…and hard to implement on busy summits.

I have used them for EU/NA events in the past and they make a big difference based on my results.

Pete
W7A SOTA

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#12

Hi John -
I’ve used both of these in the field. But not very often.
Etienne-K7ATN

Simple Two Element Yagis
http://hamuniverse.com/wb2vuo2elyagi.html

Phased Verticals for 20m

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#13

You would have better luck if you built a half-square. You need one wire a total of 67 feet long deployed as an upside down U. The rest of the square is a reflected image in the ground plane, hence the name of the antenna. Load it on the end of the wire using any matching unit appropriate for matching an endfed halfwave. It is broadside bi-direction with a very low take-off angle ideal for DX. Gain similar to a two element beam. Trim the entire antenna length for resonance on 40M. Trim the stub in the middle for resonance on 20M. Antenna will work well on 15, 20, and 40M.

73, Fred KT5X (W5YA or WS0TA on SOTA peaks)

3 Likes
#14

John,

Wire yagis definitely work. I have used large, multiband setups suspended from ropes across the tops of trees. These were for Field Day, and they improved both receiving and transmitting in the desired direction (east from my station in Wyoming).

For best results, please consider modelling your antennas using Eznec or some similar program. While the learning curve is significant, over time you’ll be able to tell where to go with your ideas. All your questions about gain, pattern, impedance, bandwidth, and what affects what will be answered right there on the screens.

Best if all, you’ll be able to sort out the truth from all the talk and advice you’ll get.

One of my favorites is the wire Bird yagi. This antenna is well-suited to hanging from trees, and one can be built with only ropes and wire.

You do need to be able to get lines over the trees, and a spinning rod and reel are ideal for that. Cast a small fishing line, pull up a larger string, and then raise the wire.

Like with all creative activities, practice and experience eventually yield amazing results!

73

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#15

I once built a phased antenna for 17m to try to work the VK0EK DXpedition. Typically for me there was a solar storm during the weekend I’d allocated to spend at my parent’s place, where there is good take off in all directions. I set up the 2 element vertical array on my parent’s lawn, I found the front to back ratio amazing! I didn’t manage a QSO with VK0EK but gosh the antenna worked a treat.

I used a piece of string to set the exact spacing between vertical antennas. I used a compass to set the correct beam heading. To switch between direction of fire meant swapping over the feeder cables to each antenna from the phasing harness which I had made.

The system works very well, but you’re tied to a single band and also only two directions of fire, 180° apart.

I have used the phased antenna system for SOTA, although it’s a bit of a faff to set up. Basically it’s two 1/4 wave verticals for 17m fed by a special phasing harness. And like I stated before, I made a spacing device from string to ensure the two verticals were the correct distance apart.

The phasing system I used is called Christman Phasing’ Google will help you find the details.

Also a good resource-

73, Colin

2 Likes
#16

Many thanks for all the replies and apologies for not replying individually to the suggestions. I need to do some reading and then some experiments. First impressions are the use of two vertical or inverted L antenna sounds attractive and may be more practical for SOTA.

I like the idea of using cord/string to maintain the spacing but stopping it all getting tangled may be a challenge! :slight_smile:

1 Like
#17

Here, I tried a 3 element inverted-V yagi built by KI6J on both 20 and 15. The boom is webbing stretched between two rows of trees on a summit. Set up time was an hour, and it was effective firing East. It immediately de-tuned when the wind arose.

Elliott, K6EL

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#18

I have a personal opinion that experimenting with antennas is closer to an art than a science. There are countless attempts on the internet for people to quantify the performance of a given antenna. Most of these would never get past a peer-review for a scientific journal (yes, I know we are amateurs). Building something and putting RF into it is a good start. However, creating near perfect conditions for a true comparison is extremely difficult. Last year I came across a C-pole antenna. I particularly liked this work done on this blog post to do some field testing/comparison. However, even these results bring up more questions.

I’ve built a fair few antennas in the past few years. I don’t have enough space at home for a reference antenna that will not interfere (wavelengths separation) with the test antenna. So I have to quickly change over antennas instead. Never quick enough to get a true A/B comparison. I’ve still never convinced myself (rbn, pskreporter etc.) that my 80m 1/4 wave inverted L with 40 radials on the ground works better than my 110 ft doublet. Contests during the winter time always seem to be during a storm. As a result, in my home setup, my doublet is mechanically much more stable and survives winter storms, so tends to be my favoured antenna to be installed and the 1/4 vertical (and radials) stay in a box in the shed. In this instance, the best antenna is the one that doesn’t break in a storm.

I think the mechanical aspects of a portable antenna are very important. The theory tells you about wavelength spacing and phase angles etc. How does that work out to be constant in 25 kt gusting wind? How does undulating summit terrain affect it?

You need a summit that is suitable for installing (more) complex antenna systems. So if you have some sort of summit constraint, why not constrain yourself to picking a summit with a good geographic profile and stick with a simple antenna?

For portable use (in typically UK weather), I think there is a strong argument for more power (assuming you are starting QRP) instead of a more complex antenna if you want to increase your chance of being heard by a chaser (assuming the mode is kept constant).

73 Matthew M5EVT.

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#19

I can’t disagree with anything you say. My only response is I started this Thread against the background I’m currently on holiday in France and was limited over the weekend to a specific low, rounded summit within walking distance of the campsite and started wondering how I might have done better trying to reach VK/ZL with what have with me.

Naturally the discussion has broadened to wider aspects of how to create a better antenna - the Holy Grail of SOTA activators!

But I like your point the best antenna is the one which stays up when the weather turns bad. It’s a bit like the old saying about which are the best binoculars or camera to buy. The answer is the ones around your neck when you need them. :slight_smile:

1 Like
#20

Hi John,
You never answered my question about speech compression. Given the fact that you are on holiday with limited options, this (rather than the antenna) might make just enough difference to get those DX stations that you are trying for. Does the radio that you have with you have a built-in speech compressor? if so have you got it turned on? (Note for stomger stations you might want to turn it off again for better quality audio reports).
73 Ed.