Strange Effect on Vertical Signals

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John Faulkner, Skegness
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Strange Effect on Vertical Signals

Post by John Faulkner, Skegness » 06 Nov 2014 00:36

In a reply to Nick's post in this thread, viewtopic.php?f=53&t=337" onclick="window.open(this.href);return false; Dave B asked the question ...
daveB wrote:The latter is an subject that warrants separate investigation - why do vertically polarised signals (apparently) 'suffer' from polarisation changes so much more so than horizontally polarised signals?

Rgds

David
Sittingbourne
This is something which has fascinated me since moving to Skegness. I have been able to hear vertically polarised signals settling in well on a horizintal antenna. Furthermore, the signals can be as good as they are if the antenna was vertical! The polarisation of the signal must be flipping, perhaps due to the properties within the troposphere.

Is this something which only happens on the coast and to signals which have travelled across water? I don't hear horizontal signals on a vertical anntenna in this way.

With the FM5 on the roof of the previous QTH, I would only ever hear a boost in vertical signals when conditions were well below average. I assumed this might be because the horizontal signals were greatly reduced in strength and thus were less likely to be strong enough to block out the verticals. Then this ...

Just before moving to the current QTH I experimented with an FM3 and an FM5 only one metre above the garden because the rooftop 9.2 was receiving a lot of noise. I found that the regular vertical signals from Holland and Belgium would suddenly leap up out of the noise and push through MUCH better then they would have done on the vertical 9.2. They were often fully quieting and even gave up the RDS ! I noticed this same thing when we moved to the current QTH when I installed a temporary FM5 on the garden, also one metre above the garden. But get this ... I recently lowered the 9.2 due to high winds and found this too had the effect of boosting the Dutch verticals, but all the time and not just during lesser conditions. I now wonder if there is some interraction between the antenna and the roof or ground. I will lower the antenna further still if we have any winter storms. I am even tempted to try this now as I can't wait to see what effect this has. (Lowering the 9.2 reduces all the ground wave signals, but the skywave signals remain largely unaffected.) I am tempted to put up a temporary FM3 or FM5, again at one metre above the ground. Why wait until winter?A

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a2c39a
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Re: Strange Effect on Vertical Signals

Post by a2c39a » 24 Dec 2014 21:06

Interesting John!

However, with my (almost) vertical (aV) and my horizontal (H), 4 element, 3m yagis, in my loft, I have quite the opposite effect!
My aV yagi hears everything (vertical or horizontal). On horizontal, French signals, there is usually a fall in signal level when I switch from aV to H yagi.
Even my weak but regular Germans on 88.0 are slightly stronger on my aV yagi than they are on my H yagi.
It is only when there is a significant lift into Germany that the H yagi seems to produce better reception than my aV yagi.

I have pondered this strange phenomon for some time.
Both yagis are in the loft of my chalet bungalow with steep pitched roof.
Is it because the metal butterfly ties in the end wall have some strange polarisation twising effect.
Is it because the aV yagi is strapped to the roof timbers, almost but not quite vertical. The H yagi is mounted clear and fully horizontal.

Since reading your post, I am now wondering what strange effect is twisting your vertical signals horizontal but my horizontal signals vertical!!??

An Enigma (oh, no, that's in my Dreambox)!!

Best wishes, John.

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Re: Strange Effect on Vertical Signals

Post by John Faulkner, Skegness » 26 Dec 2014 14:04

This is really interesting John.

I know antennas will not behave properly when in confined spaces, i.e. in a loft, but it sounds like your experiences go beyond this.

I remember Julian and myself on top of a Welsh mountain with two beams. They were a good 20 or 30 metres apart and were still influencing each other.

An antenna at one metre above the ground here exhibits good performance in both planes. The higher the antenna the more defined the two planes are. Vertically polarised signals NEED a vertical antenna when my mast is at fill height, but the vertical signals get through better when the mast is lowered. There isn't a lot of difference in the height of the mast in either case, probably only two metres, but the difference is very noticeable.

These observations are slightly different to the one I started the topic with, i.e. the physical height of the antenna as opposed to conditions. Just another anomaly to throw in :)

I think our physical surroundings, neighbouring properties, trees, etc., play more of a part in the characteristics of our antennas than we think.

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a2c39a
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Re: Strange Effect on Vertical Signals

Post by a2c39a » 26 Dec 2014 20:18

I'm sure you are correct about the surroundings influencing the behaviour of antennas, John.

I have quite a number of antennas in a relatively small space in the loft. Before the 3m 4ele aV yagi was installed (lashed to the steeply sloping roof timbers, initially with Duck tape, until it fell down, now with twin & earth!), there were a number of UHF antennas in the centre of the loft. The 3m aV has behaved in this way all the time. I added the horizontal, expecting to improve reception from Fra/Ger but it didn't hapen. I checked and checked the H yagi as I thought there must be a connection problem. I found my switch box was poor and have replaced it but direct connection of the H still made no real improvement.
As I have changed and improved my reception on other bands, I have change the number and size of the UHF antennas but this has had no noticeable affect on the aV yagi performance.

The elements of the aV yagi are only a few inches from the underside of the roof tiles!
The H yagi is relative clear all round.
You would expect the aV yagi to be detuned by it's close proximity to the roof and give poor performance. Instead it gives very good performance for an aerial mounted in a loft on both vertical and horizontal signals!!

As a vaguely related problem my close proximity to Tac gives me, I do have to be very careful with aerials, elements and any other pieces of metal in my loft!!
All metalic joints must be very good solid low resistance joints. Any poor joints will give intermod.
I found this out in TVDX days when I came across various strange signals/problems with my loft mounted Band III yagi.
If any two pieces of metal touch (just touch not deliberately bolted together) in my loft they produce intermodulation products (by the 'rusty fence' effect) from the Tac Band II signals! The intermod products are then radiated by the pieces of metal and received by the aerials close by.
Also, if I use a small linear psu (what idiot would use an smpsu close to his antennas) mounted in the loft to power a preamp then I must place 1000pF capacitors across the diodes in the rectifier bridge or it produces 50Hz+Tac (+more Tac or Tac x N) intermod products, which it radiates, again for the nearby aerials to receive.
Yes, the levels of Tac Band II in my loft really are that strong!
If due to birds, general vibration or poltergeist, some of the pieces of metal in my loft are moved into contact then up spring intermod products and I have to search the loft until I find and separate the offending items!!

Best wishes, John.

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Re: Strange Effect on Vertical Signals

Post by Tim Bucknall » 26 May 2015 17:42

sorry to drag the thread off topic, but your Tac problems remind me...

I live 9km from Jodrell Bank, when its pointing at Tacolneston, if it happens to be raining i hear the tac mixing product on 87.5!
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