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Old 19th Aug 2017, 10:36 am   #21
stevehertz
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Default Re: Shorten FM aerial elements

Quote:
Originally Posted by GMB View Post
At VHF a balun can be done quite trivially.

So a clamp-on ferrite near the feed point is a simple solution.
Better is to make a small loop of the coax so it runs more than once through the ferrite clamp (now needs a bigger hole). But not too small a loop or you damage the coax.

I usually put another clamp balun a bit further along the coax, where it has got well away from the aerial itself.

If you have any doubts about the tuner's balance requirements (as some were meant not for coax but twin line) then put another one down near the tuner.

This should then ensure that it is the aerial and not the feeder that is doing the work. But of course, if the aerial is inappropriate as yours sounds it is (on DAB) then this may just make it worse as the coax outer acts as a great big vertical wire.
Thanks, but sorry to say your well meant instruction on how to build an aerial balun has gone over my head! I'm simply not over familiar with a lot of the terminology/items named, although I'm sure it's all quite simple in practical terms. I'm afraid I'm the sort of guy who needs step by step instruction on how to build things, especially 'aerially' things. Plus I don't have 'a' tuner, I am constantly swapping and changing my vintage hifi receivers. Mostly they have 300 ohm balanced and/or 75 ohm coax sockets/connections.

How much difference would adding this balun make anyway? I know from experience that when it comes to aerial theory, what 'it' says you must do, and what works fine in practise are two very different things!
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Old 19th Aug 2017, 10:56 am   #22
G8HQP Dave
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Default Re: Shorten FM aerial elements

Antennas do what correct theory says they will do; if not, things like NEC simulation would be useless. The problem is that people misapply oversimplified theory, get the wrong answer, erect an antenna and misunderstand whatever 'measurements' they make, then annouce loudly that theory doesn't work.

A 'clamp-on ferrite' is an object you can buy. You clamp it onto the feeder cable. If the hole is big enough you can form a loop of feeder through it. It won't be perfect, but it will be better than nothing.

If it makes reception worse, then you know that your real antenna was the feeder.
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Old 19th Aug 2017, 11:27 am   #23
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Default Re: Shorten FM aerial elements

Ok, looking for 'clamp on ferrite' on Ebay brings up many suppliers of these items, described as noise filters: http://www.ebay.co.uk/itm/2x-10Pcs-C...8AAOSw42dZJ96X

So this ferrite ring that limits the frequency (noise) passed through the coax feeder also acts to better match the feeder to the aerial?
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Old 19th Aug 2017, 6:37 pm   #24
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Default Re: Shorten FM aerial elements

For a Balun on my homemade VHF Dipole I just made five loops of the coax into a coil. I did this just before it enters the connector on the dipole. Works just fine.
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Old 19th Aug 2017, 7:49 pm   #25
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Default Re: Shorten FM aerial elements?

I'm pretty sure a lot of this has already been discussed in an earlier thread, along with solutions, but I can't seem to find it.
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Old 19th Aug 2017, 10:43 pm   #26
Synchrodyne
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Default Re: Shorten FM aerial elements?

This might be the thread that you're looking for:

https://www.vintage-radio.net/forum/...d.php?t=124170

The W.T. Cocking/Wireless World article mentioned therein is a key reference, worth posting here I think:

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Also, quite recently on this forum was repoted a case where the use of a balun cleared FM reception difficulties that were initially thought to be due to problems with the receiving equipment; see: https://www.vintage-radio.net/forum/...d.php?t=138652.

Cheers,
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Old 20th Aug 2017, 6:39 am   #27
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Default Re: Shorten FM aerial elements?

Thanks guys. The main thing that I have gleaned from that lot - as has been the case in the past wrt aerial issues - is that people seem to have greatly different views on aerial theory, and on what works best in practise! It is so confusing

Saying that, as my dipole is a balanced 75 ohm one, and my feeder is 75 ohm unbalanced coax, it would appear that matching the two using a '75 ohm to 75 ohm balun' would effect an improvement. So, can we start from scratch and even though such a beast may have already be described amongst the previous comments and links, could someone point me in the direction of this life saver please?
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Old 20th Aug 2017, 10:27 am   #28
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Default Re: Shorten FM aerial elements?

Don't confuse voltage balancing (what you will see in a lot of literature) with the more practical objective of balancing the current flow to avoid the feeder helping. A lot of what is written about baluns is seriously unhelpful, which is why I did not post any links to it!

What is usually needed is to make sure that there is no current flowing along the outside of the coax. A simple ferrite clamp makes the outside of the coax look rather high impedance (but only say a few hundred ohms) so encourages any rouge current to go up the inside, where you want it.

You are trying to make the coax not be an aerial. This is exactly the same reason you see ferrite clamps used on data cables - you don't want these being aerials or they will transit their contents to the world!

What the articles on voltage baluns (the little transformers) omit to mention is that while they nicely translate balanced voltages to unbalanced voltages they only do this if they were exactly balanced/unbalanced in the first place. Real world aerials are never that good!
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Old 20th Aug 2017, 1:16 pm   #29
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Default Re: Shorten FM aerial elements?

I have a home made FM broadcast aerial mounted on my chimney. For various reasons (too many antennas already there!) I had to mount it horizontally. I checked with a spectrum analyser while on the roof to get the correct directional orientation and discovered that the signal received was a little better horizontally anyway. So it may be worth trying that with yours too if you are going to be adjusting it anyway. My dipole doesn't have a balun. It just connects straight to the coax, and uses thin aluminium rods bought from a DIY store.
Alan.
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Old 20th Aug 2017, 1:43 pm   #30
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Default Re: Shorten FM aerial elements?

I need to receive signals from all directions where I live and given my broad tastes and desire for lots of stations. So horizontal does not work for me!
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Old 20th Aug 2017, 2:01 pm   #31
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Default Re: Shorten FM aerial elements?

Multi element Yagi with rotator, or is that a bit over the top for your requirements?

Frank
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Old 20th Aug 2017, 2:24 pm   #32
G8HQP Dave
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Default Re: Shorten FM aerial elements?

Quote:
Originally Posted by stevehertz
So this ferrite ring that limits the frequency (noise) passed through the coax feeder also acts to better match the feeder to the aerial?
Yes and no. The ferrite reduces common-mode currents, and so stops the feeder appearing in parallel with one side of the dipole (and so being part of the antenna). This may affect the impedance matching too. Ferrite does not single out noise (for that you would need a Maxwell demon) but adds common-mode impedance to all signals within a broad frequency range.

Quote:
The main thing that I have gleaned from that lot - as has been the case in the past wrt aerial issues - is that people seem to have greatly different views on aerial theory
In any area of science there can be lots of wrong theories but only a few right theories. It is unfortunate that probability thus ensures that uninformed people are more likely to choose a wrong theory, as there are more of them to choose from.

Quote:
So, can we start from scratch and even though such a beast may have already be described amongst the previous comments and links, could someone point me in the direction of this life saver please?
The ferrite already mentioned fulfils this role, by acting as a current-mode balun.
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Old 20th Aug 2017, 6:17 pm   #33
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Default Re: Shorten FM aerial elements?

Thanks Dave, at last, there's light at the end of the tunnel! So, I'll purchase said noise reducing clamp or ferrite bead (aka 75 ohm balanced to 75 ohm unbalanced matching balun) and I'll be able to receive anything FM within 200 miles - right?
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Old 20th Aug 2017, 6:32 pm   #34
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Default Re: Shorten FM aerial elements?

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Multi element Yagi with rotator, or is that a bit over the top for your requirements?
Frank, I've had multiple element yagis in the past (not here), but considering that the cost of shortening my vertical dipole involved mildly blunting the teeth on a junior hacksaw blade to cut a couple of aluminium tubes, then the cost of a multi element yagi, new stronger mast, aerial rotator (c/w downstairs controller), and an aerial rigger for a few hours is er, slightly above budget by several hundred pounds! But thank you
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Old 20th Aug 2017, 7:30 pm   #35
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Default Re: Shorten FM aerial elements?

I thought I may have been over the top with that idea

Stopping currents in the coax outer braid is worth a try and and easy thing to do, only way to find out if it helps is to try it. Not many domestic FM aerials have a balun, in fact the TruMatch UHF TV aerials were designed without them, I think there was also VHF FM aerial available at one time with the same technology. Probably long gone now. Antiference I think made them, they put out lots of info how why they didn't need them, J-Beam on the other hand advertised that their aerials used a balun.

Frank
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Old 20th Aug 2017, 7:40 pm   #36
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Default Re: Shorten FM aerial elements?

J-Beam/Jaybeam did indeed use a balun at the connection-point. On their Band-II reception antennas and their high-band PMR / amateur-radio [2-Metre] variants.

The 'Balun' was a winding of high-voltage-insulation rated solid-core wire around a flat aluminium 'tab' that was connected to the feedpoint.

I never really understood how it was supposed to work, and generally found that for transmitting duty the antennas made by the German company "Kathrein"

https://www.kathrein.com/

worked better.

[Alas Kathrein have followed the masses and withdrawn from the legacy VHF-radio comms market]
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Old 20th Aug 2017, 11:25 pm   #37
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Arrow Re: Shorten FM aerial elements?

In the distant past, whenever I've erected a simple balanced half-wave dipole and fed it with unbalanced co-ax., I've always installed a quarter-wave Pawsey stub at the point where the co-ax. inner joins the dipole.

Theory: Since the 'other end' of that stub is connected to the outer of the co-ax. and that stub is a quarter-wave long, the end of it that connects to the dipole presents a high impedance to currents at that point. Hence, currents cannot flow down the outer of the co-ax.

Al.
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Old 21st Aug 2017, 10:17 am   #38
G8HQP Dave
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Default Re: Shorten FM aerial elements?

The J-beam balun was essentially a halfwave of transmission line, constructed by winding an insulated wire on an aluminium plate. It would have been complicated by coupling between adjacent turns, but they obviously got it to work OK. It may be that they exploited this coupling in some way.

An alternative way of looking at it is to regard it as a quarter-wave balanced line, shorted at the far end. Somewhat unusual line though, as it is a twin line with an adjacent ground - sort of a balanced microstrip. In either way of looking at it, a signal applied to one wire appears at the other wire with inverted phase.
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