40 meter rotatable dipole questions

630m (472 kHz) - 10 m (29 MHz) antennas, propagation, operating, etc
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40 meter rotatable dipole questions

Postby VK3XL » Fri May 13, 2016 11:21 am

Hi all, as the sun spots slowly disappear the higher HF frequencies are becoming more challenging. Although I have been licenced a long time (1979) I have not spent much time on 40 meters. I have various dipole type antennas on 40, but Im looking for something that should be better for DX on 40.
There are various types of rotatable dipoles available commercially, but as I have more than my fair share of Scottish blood in me I dont want to spend that much on an antenna. I also enjoy a challenge so I have decided to have a go at making my own.

So the design criteria that I have is as follows.

Elements made from aluminium tube.
Overall length approx 10m.
I intend to mount the antenna above a 3 el Triband yagi at about 15m above the ground.

There are designs on the net that can be summarised as below.

1. Shortened dipole with capacity hats at the end of each half of the dipole. Fed directly with coax.

2. Loading coils placed approx 1/2 way between the centre and the end of each dipole element.Feed may need some impedance transformation to achieve low VSWR

3. Loading coils placed approx 1/2 way between the centre and the end of each dipole element with capacity hats at the end. Feed may need some impedance transformation to achieve low VSWR

4. Centre loaded shortened dipole fed with the coax braid connected to the mid point of the loading coil and the centre conductor connected at a few turns away from the centre at the lowest VSWR point.

I dont have any experience in antenna modelling and I know I should learn, but being time poor I thought I would ask here for opinions on which may be the best approach.

I dont have any problems making loading coils etc as I have a fairly good engineering workshop.

So now to my questions.

1. Are 40 meter rotatable dipoles worth the effort as DX antennas when compared to a wire dipole at 10m above the ground?
2. Which of the above dipole configurations are likely to be the best performance : difficulty to construct ratio?

Thanks for taking the time to read this and any advise on topic will be gratefully received.
73 Mike
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Re: 40 meter rotatable dipole questions

Postby VK3LU » Fri May 13, 2016 2:23 pm

Mike,
I have emailed something that might be of interest.

73 Nev
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Re: 40 meter rotatable dipole questions

Postby VK2DUX » Fri May 13, 2016 3:28 pm

VK3LU short of the email contents being highly classified would you care to share it with us? I also have an interest in such a project and no doubt several others will as well.

de john vk2dux
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Re: 40 meter rotatable dipole questions

Postby VK3XL » Fri May 13, 2016 4:42 pm

Which email address did you send it to Nev. I havnt got anything yet.

Sent from my SM-G920I using Tapatalk
73 Mike
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Re: 40 meter rotatable dipole questions

Postby VK3LU » Fri May 13, 2016 5:01 pm

The Yahoo one. I hope thats the right one as it worked last time.
John, I will attach the info.
Nev
page1.jpg

page2.jpg
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Re: 40 meter rotatable dipole questions

Postby VK2DUX » Fri May 13, 2016 5:26 pm

hank you for posting Nev, A good read.
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Re: 40 meter rotatable dipole questions

Postby VK3ALB » Fri May 13, 2016 6:45 pm

I think you should make the effort to learn antenna modeling. Even at a basic level there is much to be gained from modelling your design before you even step out in the back yard. You have the opportunity to see how your antenna will perform in an ideal world which is a good starting point. Of course, you'll then add trees, sheds power lines, other antennas into the mix once you get you antenna in the air but at least you have a known starting point.
Be reasonable - do it my way.
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Re: 40 meter rotatable dipole questions

Postby VK4WDM » Fri May 13, 2016 9:31 pm

Up to a year ago I was using a horizontal delta loop on 40m. Highest point was only 8m and the other two 6m. I found to be an excellent antenna on 40m with much lower noise than my multi-band vertical. It was also resonant on 15m as you would expect and could be used on the other bands with a tuner. Unfortunately I can no longer use it because I shifted my shack to another part of the house, but I sure do miss it. :cry:

73

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Re: 40 meter rotatable dipole questions

Postby VK3XL » Sat May 14, 2016 1:45 pm

I agree Lou, I should learn to use an antenna modeling package and I probably will in the future. My main reason for this post was to see if anyone had experience with these types of antennas before I invest time and effort into the project.

Wayne, I have also thought about a delta loop for 40, but that would have to be set in a fixed direction and due to the positioning of terre and the shed in the back yard it would probably only favour the north east direction. The south west would be mostly blocked by the shed. Therefore the rotary dipole at 16m above the ground would solve most , if not all of those issues.
73 Mike
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Re: 40 meter rotatable dipole questions

Postby VK3MEG » Sat May 14, 2016 8:55 pm

i have a vertical 40m delta loop you get one corner of it to 16m it will do great the top of mine is @ 10 m and the bottom @ 3m it rocks pretty hard eu workable ssb and cw the us south america well worth the effort. i also have a 40m dipole @ 10m the loops is 1-2 s points better. and has a good recieve. i have worked 113 countries on 40m and i dont use digital modes i use ssb and cw 100w. from western sahara to market reef look at the delta loop.
or look at running 4 slopers off the tower using the yagi as a capacetance hat i run an 80m sloper with good results 40m also worked well radials help there as well arrl antenna handbook has a great section on slopers they offer directivity and some front to back.
cheers
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Steve
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Re: 40 meter rotatable dipole questions

Postby VK3HJ » Sat May 14, 2016 11:34 pm

I support your proposal for a rotary dipole.

Several years ago, I started noticing many DX stations using rotary dipoles with good results, surprisingly superior to their fixed wire dipoles.

A few years ago, I knocked together a rotary dipole, and used two 7 metre telescopic fibreglass poles. I had a couple of 30 m traps, so set it up for 30 m and 40 m. It needed some inductive loading between the feed point and the traps. Feed impedance was acceptable on both bands. Performance was quite good at a height of about 15 metres. The rotary dipole showed a useful azimuthal pattern, with good rejection off the ends of the element.

I have since replaced this with beams for both bands. The 2 element yagi for 40 m uses both loading coils and capacity hats on the elements, and a shunt inductance at the feed point (Comantenna). Driven element is 11.6 m, cf ~20 m. Shortening the elements complicates them and narrows the SWR bandwidth, but this is a trade-off against full sized elements, which are rather long. I chose to tune the antenna for the CW end of the band, and use matching in the amplifier to work the phone end of the band.

73,
Luke VK3HJ
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Re: 40 meter rotatable dipole questions

Postby VK4WDM » Mon May 16, 2016 8:24 am

Last night 40m signals to the USA were marginal and there was very strong QRM from Asia, so a directional beam of some sort would have been very useful.

Not enough space for a rotatable dipole but a mag loop is back on the agenda.

73

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Re: 40 meter rotatable dipole questions

Postby VK3XL » Mon May 16, 2016 10:50 am

Thanks Gents for all your replies, I am still going down the rotary dipole track rather than any fixed wire antenna simply because I want to be able to orient the antenna in the direction of the DX. For various reasons this is not possible at my place with wire antennas without putting up horizontal delta loops in multiple directions.

Thanks Luke for your reply, this gives me more motivation to progress this further. Due to family commitments I will not be progressing with this project for a few weeks, so I suppose I should take Lou's advise and get the hang of antenna modelling while I am not able to get into the workshop.

Again thanks for all the replies.
73 Mike
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Re: 40 meter rotatable dipole questions

Postby VK3XL » Mon May 16, 2016 11:19 am

By the way if there is anyone looking to do a similar thing to what I am planning here are a few links you may find useful

This is my preffered option at the moment
http://n6jv.com/40meterdipole.html

Other useful links
http://hamantennas.blogspot.com.au/2011 ... eters.html
http://www.flashwebhost.com/circuit/shortant.php
http://www.k7mem.com/Electronic_Noteboo ... l#Cust_Dim
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=23263

yet another approach is to turn your HF yagi into a rotary dipole by loading the boom.
http://lists.contesting.com/archives//h ... 00731.html
73 Mike
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Re: 40 meter rotatable dipole questions

Postby VK3IH » Mon May 16, 2016 4:43 pm

no-one yet has mentioned a 40m vertical mounted 3-4m up at the base?
Paul VK3IH / G3ZDR South Gippsland Victoria
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Re: 40 meter rotatable dipole questions

Postby VK3LU » Mon May 16, 2016 5:11 pm

Probably because Mike is really only interested in a rotateable dipole. :shock:

73 Nev
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Re: 40 meter rotatable dipole questions

Postby VK3HJ » Tue May 17, 2016 1:49 am

A vertical monopole offers no rejection in any direction. A horizontal dipole can provide some rejection off the ends.

Another antenna I experimented with was a vertical array - Bobtail Curtain. I oriented this to reject the crap to the north, and provide some gain in two directions in which I was interested - North America and Africa.
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Re: 40 meter rotatable dipole questions

Postby VK3LU » Tue May 17, 2016 4:36 pm

VK3HJ,
How did the Bobtail Curtain perform?
I have always been intrigued by that type of antenna. Alas I don't have the room.

73 Nev
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Re: 40 meter rotatable dipole questions

Postby VK3YE » Tue May 17, 2016 9:53 pm

VK3LU wrote:VK3HJ,
How did the Bobtail Curtain perform?
I have always been intrigued by that type of antenna. Alas I don't have the room.


I recently made one for 432 MHz and it was clearly better than a dipole when receiving a beacon. A demonstration is below.

https://www.youtube.com/watch?v=FgJVuEq5Its

Another possibility is the half square which requires half as much yard space as the bobtail. Many who can fit in a dipole can also fit in a half square provided height is sufficient for the 1/4 wavelength vertical elements (plus some clearance from the ground). Haven't done a head-to-head comparison with other antennas but seems to perform well.

https://www.youtube.com/watch?v=a8Zh1_Mntz0

https://www.youtube.com/watch?v=n_lmzHe40O0
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Re: 40 meter rotatable dipole questions

Postby VK3HJ » Wed May 18, 2016 1:25 am

The Bobtail Curtain performed very well. It certainly worked well broadside, compared to a large doublet antenna. Rubbish from the north (off the end) was noticeably attenuated.

As Peter noted above, a 2 element broadside array, known as a Half Square works nearly as well.

But as you already have a tower, a rotary dipole would probably be easier to erect.
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