Double Sized Flag Antenna - Not impressed

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John Faulkner, Skegness
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Double Sized Flag Antenna - Not impressed

Post by John Faulkner, Skegness » 20 Apr 2016 18:07

A lot of medium wave DXers use a double sized flag antenna.

So, I was out in the garden today, performing my annual cutting down of the conifer tree ceremony. It overhangs into our garden and stops me from turning the 9.2 to the south-west and further clockwise. Then to my next antenna job, putting up a permanent South American flag for the summer.

Before I did this, I thought I would double-size it and do some comparisons. Since there is nothing to null from the north-east anymore, I had to reverse the flag to set up the null. First with the regular flag of 10 ft up, 25 ft across, 10ft down and a 25 ft base wire to connect from the transformer box to the resistor box. In metric that's roughly 3 metres by 7.7 metres.

BBC Radio Leicester on 837 is a decent signal here, so I used that to set up the resistor and obtain the deepest null, almost totally removing it!

Then to increase all dimensions by two. Unshiftable BBC Radio Leicester! Only the slightest reduction in signal.

Julian - remember the double sized flag you tried on your garden? That too didn't have a lot of nulling power on it either. What needs to be done to achieve the same degree of rear nulling as the standard sized flag?

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Re: Double Sized Flag Antenna - Not impressed

Post by John Faulkner, Skegness » 21 Apr 2016 19:35

I carried out a few more extremely non-technical experiments with flag antennas this afternoon.

Firstly, I wanted to see if reception / performance could be improved by changing the thickness of the wire. I tried some really flimsy thin stranded wire to begin with, taking a few signal measurements across the band, then comparing with my original and slightly thicker wire. Then I tried some really thick stuff which barely bends. While I don't have the exact thickness of the wires, there was quite a variation between the tihin stuff and the thick stuff!! The think stuff is just 'too thin' to be of any real use and would probably snap in a gust of wind. The thick wire just wouldn't flex easily. I think that says it all. The results were the same: The same amount of 'pick-up' and no difference in nulling power to the rear. (I don't think the colour of the insulation makes a difference! ;) )

I was careful to check signals at several points across the medium wave band to get a more complete picture.

Next I simply increased the height of the vertical sections of wire by one metre (from 3 metres to 4 metres) ... and NOT the distance between them. I repeated the same comparisons. Bingo! A small but useful improvement! Signals were a few dB better, both in nulling ability and in forward gain - about 3 to 4 dB, making 6 ro 8 dB improvement in the front to back ratio. I am pleased to say that background noise levels remained unchanged around -120 to 125 dB.

These comparisons were carried out late afternoon and I think we before any night time atmospheric effects had time to take a hold. I used local-ish transmitters anyway and the comparisons were only spaced apart by five minutes or so.

I'll try doubling the height of the vertical sections shortly, but keeping them at the same 7.7 m spacing.

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Re: Double Sized Flag Antenna - Not impressed

Post by satnipper » 21 Apr 2016 22:05

Around 12x4m is optimal for Flag antennas. I don't think many dxers user "double-Flags" - think you mean "double-KAZ"?

Stuart

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Re: Double Sized Flag Antenna - Not impressed

Post by John Faulkner, Skegness » 22 Apr 2016 00:27

I don't know what a double Kaz is. I have heard of the SuperKaz.

As for the flag, I know there are a few successful MW DXers who use the 12m x 4m flag to good effect, but it doesn't work for me. I am guessing that the transformer needs different numbers. Unfortunately, I don't have the details for these and I daren't rip one apart to check as I know it won't work again if I do. I have a 0% success rate when it comes to building those tiny little binocular transformers.

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Re: Double Sized Flag Antenna - Not impressed

Post by John Faulkner, Skegness » 22 Apr 2016 13:46

After some research, it seems the value of the transformer, winding ratios and terminating resistor values I use on the smaller flag are all fine for the larger flag too. To be fair, there are some differences when used with the larger flag, more so to do with the resistor setting, but nothing which would be a problem for either size. So I still don't know why the larger flag has so much less rear end rejection.

As an alternative, I am looking into flag amplification, which I understand is a popular choice for flags.

Edited in the hope that this is more coherent .... hopefully ;)

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Re: Double Sized Flag Antenna - Not impressed

Post by satnipper » 22 Apr 2016 21:35

The Flag operates as a pair of phased verticals - hence the rear rejection. Once you make it larger than around 12x4 it ceases to operate as phased verticals and you lose the rear rejection. This is pretty well documented.

The binoc toroids are the bees-knees and the typical turns ratio is 12:4 or 12:3 - think I wrote on this on my blog couple of years ago.

David Hamilton is a flag-meister and it were he who introduced me to them few years ago.

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Re: Double Sized Flag Antenna - Not impressed

Post by satnipper » 22 Apr 2016 21:39

I'm also not convinced that a Binoc 12x4m Flag needs amplification when it's used with a SDR. May have to switch in attenuation which kind of defeats the point.

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Re: Double Sized Flag Antenna - Not impressed

Post by John Faulkner, Skegness » 22 Apr 2016 23:58

satnipper wrote:The Flag operates as a pair of phased verticals - hence the rear rejection. Once you make it larger than around 12x4 it ceases to operate as phased verticals and you lose the rear rejection. This is pretty well documented.

The binoc toroids are the bees-knees and the typical turns ratio is 12:4 or 12:3 - think I wrote on this on my blog couple of years ago.

David Hamilton is a flag-meister and it were he who introduced me to them few years ago.
Thanks for the info Stuart.

But you said 12m x 4m sized flag was optimal?

Anyway, I reconstructed the flag again this evening and it's now approximately 5m tall by 7.7m in length. It's working well. There is a little more gain while the rear nulls are the same, so the f/b ratio is improved a little more. The resistor setting for the rear null is now quite "wide" and just as deep, so no fiddly adjustments needed anymore. These dimensions are working well for me.

I hope to do some more experimentation and, fingers crossed that those noise levels stay well away. As I previously mentioned, MW was wiped out here over the middle of winter. Whatever it was must have been on its way out and eventually conked completely.

I think a flag amp, something like the RPA1, will be a good investment. As you say, a degree of attenuation might be necessary at times, but I feel sure there would be some benefits when used on the smaller flag.

It's 2250 and already some TA audio is on the band on 1280, 1400 and 1560 so something is working well.

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Re: Double Sized Flag Antenna - Not impressed

Post by satnipper » 23 Apr 2016 03:45

There is some research around which showed that once you went beyond length of 12m and height of 4m then the law of diminishing returns sets in. Can't remember where I read that - may have been ON4UN's low band bible. I think the flag then starts operating more like a loop which is bi-directional.

I have a RPA1 but have never found it useful but Paul C swears by his. So be interesting to try it.

Cheers

Stuart

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Re: Double Sized Flag Antenna - Not impressed

Post by tvdxrools » 23 Apr 2016 09:59

once you go over a certain size the flag works just like a omni loop so you just have to experiment how large you can get it before it comes to that , i always put my feed/resistor on the middle of the vertical legs as that's where i found the best match and as for baluns i mostly used a 48/11 turn ol balun ,

i have never used a amp on mw as you are not looking for s9++ signals you are looking for the best signal to noise, i am quite happy with a s2 signal with id than a s9 signal backed up with mush and if you notice on my you tube videos with the flag the signals are not strong but the signal to noise ratio is good enough to get the dx , cheers David

ps the balun is to match the coax to the flag ie from either 250 - 300 ohm down to 50/75 ohm nothing more

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Re: Double Sized Flag Antenna - Not impressed

Post by John Faulkner, Skegness » 23 Apr 2016 10:29

satnipper wrote:Can't remember where I read that - may have been ON4UN's low band bible. I think the flag then starts operating more like a loop which is bi-directional.

Stuart
That's something I hadn't thought of. There were a few DXers in the 80s who used a single turn loop. Some people would literally sit inside one as it was wound around their shack. Derek Taylor in Preston used one to good effect. I briefly tried one, suspended a few inches away from the walls, ceiling and floor and it worked quite well when phased against my smaller loop so it became unidirectional. The smaller loop was the unidirectional one.

This current flag setup, 5 x 7.7 m, is starting to look more like a square, but it seems to have a reasonably unidirectional pattern.

Now we are approaching the summer months (even though the weather tells us we're slipping back into winter) I am going to position the flag for southerly directions. I want to try Africa in the evenings and then South America overnight. I'll post about this in a new thread. I have never tried for African medium wave DX.

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Re: Double Sized Flag Antenna - Not impressed

Post by John Faulkner, Skegness » 23 Apr 2016 10:41

tvdxrools wrote:once you go over a certain size the flag works just like a omni loop so you just have to experiment how large you can get it before it comes to that , i always put my feed/resistor on the middle of the vertical legs as that's where i found the best match and as for baluns i mostly used a 48/11 turn ol balun ,

i have never used a amp on mw as you are not looking for s9++ signals you are looking for the best signal to noise, i am quite happy with a s2 signal with id than a s9 signal backed up with mush and if you notice on my you tube videos with the flag the signals are not strong but the signal to noise ratio is good enough to get the dx , cheers David

ps the balun is to match the coax to the flag ie from either 250 - 300 ohm down to 50/75 ohm nothing more
Thanks David.

I have heard that the resistor is better placed in the middle of the vertical wire. This is something I need to try.

I am always impressed by your videos showing your low noise floor, etc. It's the same when DXing mobile in electrically quiet locations. The weakest signals are suddenly so clear. They wouldn't stand a chance at home.

The transformer I have been using is 5 turns on the primary (coax side) and 22 turns on the secondary. It's thin wire and plastic coated. The cores I use are from Westlake, type 43. I have a stack of figures here for different sized flags and Beverages, used under different conditions, i.e. BOGs, wires draped along tall grass, hedgerow Beverages, etc. and for different lengths. I think I have got everything exactly as it needs to be, but you know how it is with antennas - what works in practice is sometimes different to the theory due to our surroundings.

I think future MW DXing will be done back on the smaller flag (3m x 7.7m) but with an RPA1 or similar. The RPA1 will definitely come in useful for a small flag when the noise floor is low.

DX Engineering RPA-1: http://www.dxengineering.com/parts/dxe-rpa-1
RPA-1 Reviews: http://www.eham.net/reviews/detail/5321

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