40 meter rotatable dipole questions

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

Postby VK5LTD » Thu May 26, 2016 8:39 pm

I'm considering 40m antenna options for the very reasons mentioned in the original post so this thread is timely.

I had in mind a rotatable dipole also but delta loop also a possibility.

There would be two potentially uncooperative trees for two points of the delta loop and back to a J mount on a chimney.

It might make keeping to specific lengths difficult, so wondered if just getting as much wire into the air as possible, using a balun of some description then using a tuner is viable.

The capability to rotate and be broadside to desired bearing would be very advantageous so definitely interested in the topic. Space may be a limiting factor for my suburban block.
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Re: 40 meter rotatable dipole questions

Postby VK4WDM » Thu May 26, 2016 9:32 pm

My 40m delta loop was short by about 2.5m but the internal tuner in the IC756 PRO3 still matched it easily. I used a 2:1 balun at the feed point and an 8 turn coax wound choke below that.

It also worked very well on 15m and quite well on other bands with an external tuner (MFJ Versa 2). It was fed with coax but some claim that delta loops work much better with open wire line.

It was certainly a lot better on 40m DX than the loaded dipole I am using now. I would like to put it back up, but I would have to rotate the house, and my XYL is not keen on that idea :mrgreen:

73

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Re: 40 meter rotatable dipole questions

Postby VK3XL » Fri Jun 24, 2016 10:47 am

OK the family commitments are now back to normal (what ever normal is around here) So on with the rotary dipole project. I think my first attempt will be a loaded dipole arrangement using 2 x 7M squid poles. Now the weather will be the limiting factor as to when I can get the tower down to try it out.

Thanks to all that replied. I will keep you posted on the results.
73 Mike
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Re: 40 meter rotatable dipole questions

Postby VK4WDM » Fri Jun 24, 2016 7:23 pm

I wonder how a helical wound dipole would go? I use two helical whips as a dipole for portable work on the higher bands and it works quite well.

73

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Re: 40 meter rotatable dipole questions

Postby VK3AUU » Fri Jun 24, 2016 8:34 pm

Height is the most important thing if you want to get as low a radiation angle as possible.

For a full size 40 metre dipole.
At a height of 16 metres, which is as high as a fully extended Nally tower with the supplied tube out the top, the
maximum broadside gain of about 4.4 dbd will still be at around 40 degrees and the 0 dbd gain will be at 15 degrees.
The same dipole at 15 metres will have about 6.2 dbd gain at 13 degrees.

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Re: 40 meter rotatable dipole questions

Postby VK4WDM » Fri Jun 24, 2016 9:43 pm

Hi David

10m is the height limit here unfortunately. My multiband vertical works very well on 40m but I have huge QRM from Asian stations, mainly YB, so some directivity would be welcome thus my interest in a rotatable dipole, but my backyard is very small and the turning circle is limited. Gain is not really an issue, if I can hear them I can usually work them but the hearing bit is the problem }:[

73

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Re: 40 meter rotatable dipole questions

Postby VK3AUU » Sat Jun 25, 2016 6:33 am

The problem with almost any sort of dipole on 40 metres at a height of only 10 metres is that it isn't going to be very directional and there are no decent nulls to cancel out unwanted qrm. A 2 element close spaced delta loop on a 7 metre long boom has a 20 db null off the side and 8.5 db back to front. That looks about the best I can come up with if you are really keen.

David
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Re: 40 meter rotatable dipole questions

Postby VK3XL » Sat Jun 25, 2016 7:25 am

Thanks David for your input. At this stage I want to keep the 3 el tribander up on the tower so the delta loop wont suit my setup at this time, it may in the future. I intend to put the short rotatable dipole at about 15 or 16 meters above the ground, not sure if that will make a difference to the pattern compared with 10 meters above the ground as per your post. I will see how it compares to my wire dipole at 10m above the ground once I get the rotatable dipole sorted out and up the tower.
73 Mike
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Re: 40 meter rotatable dipole questions

Postby VK3ZAZ » Sat Jun 25, 2016 10:15 am

FWIW

Its not worth commenting on these pages because no one listens and everyone has their own opinion

WDM I have WAZ #1 VK 40M I know what I am talking about and it wasn't DIGITAL Modes

As for summer es care less


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Re: 40 meter rotatable dipole questions

Postby VK4WDM » Sat Jun 25, 2016 12:28 pm

but running 100 watts waste of time on the 40M


I don't agree Steve. On 40m I run 55w digital modes into a vertical (with a good radial system) and I am very close to DXCC and WAS on that band on digital. This morning I worked into EU, NA and ZS.

I have never run any more power than 100w in my entire ham life (55 yrs) and usually that was to wire antennas. I only had a beam for about a third of that time. 238 MF and HF DXCC enties (59 on 6m) worked under this call sign (started in 1993) mostly on SSB with some digital modes in recent years. So hardly a "waste of time."

I usually abide by the old African proverb "that he who blows his own horn makes a terrible sound" but I think it is a disservice to the hobby, especially the foundation and standard holders to say that 100w is a waste of time (off soap box).

My main problem on 40m is in late afternoon and evening where there is terrible ham and non-ham QRM from Indonesia that is talked about in another thread. It is almost impossible to get decent decodes and even CW is hopeless.

The best 40m antenna I have had was a horizontal delta loop at only 8m high but I can't fit it in to the yard. I used the vertical to transmit and the loop to receive and that was a good combination. I might do as you suggest and use an inverted V if I can get the orientation right to minimize reception from the NNW.

Note the wonderful 6m summer E season they are having in the northern hemisphere. Hopefully it will be our turn in a few months time. :D

73

Wayne VK4WDM :om:
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Re: 40 meter rotatable dipole questions

Postby VK3MEG » Sat Jun 25, 2016 4:02 pm

my delta loop is now vertical and its fed with with a 1/4 of 75 ohm coax i have worked 113 on 40m only 85 odd confirmed no waz here africa is a pita for me. i have also used a slopee,inverted v, dipole and vertical i dont do digimodes very much but do do cw. and ssb only used 100w
cheers
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Re: 40 meter rotatable dipole questions

Postby VK3HJ » Sun Jun 26, 2016 12:14 am

VK3ZAZ wrote:FWIW
Its not worth commenting on these pages because no one listens and everyone has their own opinion


What are you on about Gregory? You appear to have no posts in this thread to ignore.


As mentioned before, a vertical array may be worth considering. A "half square" will fit in your backyard and you can align it to null The Crap From The North.
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Re: 40 meter rotatable dipole questions

Postby VK5ZD » Sun Jun 26, 2016 2:32 am

VK3ZAZ wrote:Its not worth commenting on these pages because no one listens and everyone has their own opinion.

Surely the whole point of a forum is for people to express their opinions...
73
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Re: 40 meter rotatable dipole questions

Postby VK4WDM » Sun Jun 26, 2016 9:22 am

A vertical array would work very well but I promised my XYL that I would try and avoid cluttering the yard with too many antennas at this QTH. The other issue is that I need to be able drop the antennas down very quickly to cope with the nasty storms and cyclones that we have up here. So the less masts the better!

I am going to try Steve's suggestion (yes we do listen Steve) of a simple inverted V. I will put two up in different directions and see how my of a null I can achieve.

I will still be watching the OP experiments with the rotatable dipole though.

73

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Re: 40 meter rotatable dipole questions

Postby VK3XL » Fri Jul 22, 2016 6:53 pm

Well I've taken Lou's advise and learned how to model antennas (at least basic ones) and have come to the conclusion that a rotatable dipole at 13 -15 meters is not going to give me the sort of performance I was looking for. So now I am looking at a delta loop fed as a vertical and I will just have to put up with it not being rotatable. I might see if I can find a way of putting up 2 at 90 deg to each other in separate areas of the yard...... more modeling required.
73 Mike
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Re: 40 meter rotatable dipole questions

Postby VK3ZAZ » Sat Jul 23, 2016 2:57 pm

FWIW
I persevered with a dipole for 20M when my array mucked up and it was really poor, put reflector behind it and it quietened down.
I have since put up a 2 EL 20M beam made up from a TH3 reduced to a aTH2 dimension which they made before TH3s and it points and works fine I cant tell the difference even though book figures say I dropped db or so.
I got it across the band by winding 20 turns of RG8 and stuffing the line down the inside of one of the tuning lines so its totally decoupled and is broad across whole of 20M.
The main consideration with modern rigs seems to be RESONANCE so u need to decide where in the band to put the 40M device its a wide band now from CW end to USA Phone end and one antenna wont cut it without line tuning.
Then your radio can run its full power and try to do it barefoot no ATU
As someone just said to me you are doing rather well on 20M with 200 watts

As for 40M
I did my 100 countries on a delta loop which was quieter until my wife got pacemaker and I found the emr levels are too high with antenna so low to ground.
With decline in cycle this next equinox will see some good 40M
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Re: 40 meter rotatable dipole questions

Postby VK4WDM » Sat Jul 23, 2016 7:27 pm

I found the emr levels are too high with antenna so low to ground.


The regulations say that we have to have safe EMR levels but council regs limit the height of our antennas so how are we supposed to comply? }:[

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Re: 40 meter rotatable dipole questions

Postby VK2CSW » Sat Jul 23, 2016 8:33 pm

VK4WDM wrote:
I found the emr levels are too high with antenna so low to ground.


The regulations say that we have to have safe EMR levels but council regs limit the height of our antennas so how are we supposed to comply? }:[

73

Wayne VK4WDM :om:


Probably something our "representatives" should be looking into.

(I am not going to hold my breath) :beer:
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Re: 40 meter rotatable dipole questions

Postby VK4WDM » Sun Jul 24, 2016 10:46 am

The other antenna worth a mention is a magnetic loop. I am making one to try out as a receiving antenna but the reports of its directivity vary.

73

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Re: 40 meter rotatable dipole questions

Postby VK4WDM » Sun Jul 24, 2016 4:54 pm

AB7CR has some detailed info on loops on his QRZ.com page.

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