Ground gain - effect of antenna height...

[VK2XV]

While understanding the principle of ground gain I am wondering what the effect is of significant antenna height. It seems from what information I can find is that something between a few wl and 10 wl is best.

Specifically I wonder what the effect would be for an antenna operating on the edge of a 70m cliff edge on 2M (35 wl) ?

[VK4WTN]

hi Steve,
I have found on 6M antenna height makes a huge difference along with location.We did a test with KH7Y earlier this year and he has I think 2 x 7 el yagis at different heights.On the bottem yagi he was 56 he switched to the top yagi and the signal jumped to 20db over 9, he then ran the two antennas in phase which made little difference on the single top antenna. This was a great learning experience on the effects of antenna height. Also Paul VK4MA who lives about 7km from here generally hears 2 to 3 s points better than stations in Hervey Bay. Most of the guys here have antennas at 10M height where Paul has his antennas at about 19M high. This again shows the advantage of antenna height.
regards Wayne VK4WTN

[VK3ZAZ]

K6STI DID SOM GRAPHS
but only for a dipole

http://www.ham-radio.com/k6sti/height.htm

So if that's what you are asking about a single yagi like I had at 25M agl it exhibited had best gain at 2-5 degrees for eme if you cant elevate.
But it was a 2.5WL yagi with a narrow aperture.

calculator for other antenna up 1 wl
https://www.rac.ca/tca/Antenna_Height_Calculator.html

I think ground gain vs height applies to EME as much as anything.
Does it not

is this what you were asking?
as for stacked heights using ionospheric prop as the signal yardstick too many variables to be conclusive especially the aperture.

better to try 2M stacks on a known path and even then the ref aerial has to be at same height..
But if you want the good oil on 6M arrays talk with W6BYA two stacks
K6QXY four stacks and dual polarity
or W6JKV 16 bay array
or Mike K6MYC four bay switch

[VK2XV]

G'day Wayne and Steve - thanks for replying.

I am fairly well across the scenarios as mentioned (stacking and common heights).

What I am interested in is the specific case I mentioned:- a 2M antenna @ 70m.

As you might have gathered, from previous moans by me, the QTH here is not optimal for VHF-up operation because of the 100-odd trees on a 3/4 acre block.

I am interested in EME @ 2M and while the trees preclude operation near the house, there is a clear view for moonrise at the back of the block from a 70m high cliff.

It is this setup that I want to get information about.

As described elsewhere, there is up to 6dB of extra directivity over the free-space pattern obtainable by ground reflections, dependent on the conductivity of the ground and, of course, the coincidence of the interference lobe with the natural beam lobe of the antenna. Literature I have read generally only describes heights of up to about 5wl. Additionally, there are vague references to ground gain falling off at higher antenna heights - but no explanation as to why. My own theory about this is that at extreme heights the reflection points can be quite a distance away (orders of kilometres) and so the reflection is less specular and more diffuse. While this means there is a reduction in the depth of the nulls in the interference pattern, it must follow that the peaks will not be as strong.

It is important for operation where the link budget is tight that some estimation of antenna gain is made - therefore it is important to get good estimate of ground gain to determine the viability of a particular setup.

The question is - what effect does the operation of a 2M @ 70m height have on the nominal 6dB ground gain ?

[VK5APN]

Steve

Google is your friend.

An extensive study and documents have been put together by ON4KHG

http://www.on4khg.be/EME_Gr_Gain.html

in particular

http://www.vk3um.com/Ground%20Gain_DUBU ... ON4KHG.pdf
http://www.on4khg.be/Ground%20Gain%20Me ... 20v2-0.pdf

Hope that helps you

VK5APN

[VK3AUU]

I have done some actual measurements on both 6 metres and 2 metres, On 6 Metres I measured around 5.5 db improvement at a take off angle of around 8 degrees with the antenna at a height of 8 metres. This was done using W7GJ off the moon with my 15 element yagi. This agreed fairly well with the EZNEC model. All extra gain was lost above about 15 degrees. It was then time to start tilting the antenna. I rana WSJT test with a station on the Isle of Mann where the moon was at around 8 degrees elevation at both ends. We certainly had a peak there but signals peaked at -27, but not quite long enough for a contact.

On 2 metres using the setting sun with a single 19 element DL6WU at a height of 12 metres I measured peaks at 3,9,15 and 21 degrees with quite deep nulls in between. This also agrees fairly well with the model. The second peak waas actually slightly better than the first in practice. The immediate foreground was a fairly flat grassy paddock.

The angle of radiation is an inverse function of the height of the centre of the antenna capture area above effective ground. Stacking does not lower the angle of radiation. If you want to compare antennas, you MUST have them at the same height.

One of the reasons why higher antennas work better is because of the lower angle of radiation and this generally corresponds with the low angle at which the incoming signal arrives. The station on the cliff top should get the whole 6 db plus something extra because of a greaater amount of incoming signal.

Ens of lesson.

David

[VK2XV]

G'day Wayne,

Google is your friend.

Yes - I know - I had already read the ON4KHG documents...

http://www.on4khg.be/EME_Gr_Gain.html

This a quote from that document...

The antenna must not be too high up in the air. This, for two reasons, the first is that if it is too high, the ground gain lobes will be narrow, even too narrow to complete a JT65 QSO over a single lobe (with my antenna 17m agl, I can only make one QSO over one lobe, i.e. around 6 min). The second reason is that the higher the antenna, the furthest from the antenna the building of the ground gain lobes and hence, the higher the likelihood the ground not to be flat.

...
and again...

One outcome of this analysis is the conclusion that placing an antenna as high as possible up in the
air is not always an absolute requirement. Indeed, the higher an antenna, the further away from it
the Ground Gain lobes will build and, at some sites, the higher the likelihood that the ground is
cluttered with vegetation and/or buildings (attenuating the magnitude of the reflected wave). In this
case, is it worth losing 6 dB of Ground Gain by winning a few (tens) kilometres of short-range radio
horizon ? All is a matter of trade-off..

Soldiering on I downloaded the simulator file (http://www.on4khg.be/Ground%20Gain%20Ge ... 20File.zip), but found when loading into OpenOffice it locked up my computer requiring a reboot (Task Manager couldn't terminate it) - never had to reboot before for anything (we can argue whether this is the fault of the file or OO...). In any case it only theoretical and doesn't take into account the real effects of distant reflection points. It would only show very fine interference lobes - but as I have mentioned previously - I already know the theory.

I guess there is no answer to my question unless I can google an EME-er with a 70m tower...

[VK3ZAZ]

Height is everything.
IMHO
As for 70 metres no experience except for work QTH which runs FM 3 el stacked yagis half horizontal zero and half with down tilt to cover near region.

EME
Most eme people I have seen have arrays close to the ground
including Mike K6MYC in Fresno the M2 Guru and Jimmy W6JKV the eme expert.

If u r going to 70M with LDF5/50 or 21/4 inch u r obviously filthy rich so should be able to research the knowledge u need.
we are but humble amateurs.

[VK2XV]

I have done some actual measurements on both 6 metres and 2 metres...

G'day David,

Thanks.

Yes - I understand all this - but I am after an answer to a 70m high 2M antenna.

Could you repeat the experiments after cranking up your tower to 70m ?

[VK3ZAZ]

75 feet
50 watts +47dbm

2014-12-22 04:38 VK3ZAZ 144.490504 SNR +0 0 QF12ag +47 50.119 VK2KRR QF34mr 524 km

and 1 watt +30 dbm

2014-12-22 04:42 VK3ZAZ 144.490505 SNR -16 0 QF12ag +30 1.000 VK2KRR QF34mr 524 km


17 db drop in power produced 16 db drop in snr

Free space for sure I imagine going from 25M to 75M would prove nothing except cost me about $500 in cable and increase attenuation even then.
dropping it down to 10 and 5m would be terminal

Correct me if I am wrong?

[VK2XV]

Correct me if I am wrong?

G'day Steve - can't answer this as I struggle to see how this is related to the topic...

[VK2XV]

If u r going to 70M with LDF5/50 or 21/4 inch u r obviously filthy rich so should be able to research the knowledge u need.
we are but humble amateurs.

G'day Steve - as it is on a 70m cliff why do I need to be rich...

[VK2XV]

Found this interesting exchange of opinions and practical experience...

http://www.dxmaps.com/discuss/polarization.html

...it's about polarisation - but feeds into ground gain effects...

[ZL1RS]

An article I wrote for NZART Break-In magazine in 2010 combines theory and practical results of 2m antennas at various heights ... including at 50m above the ocean.

http://www.qsl.net/z/zl1rs//images/groundgain.pdf (1.3MB)

While specific measurements of signal level vs. moon elevation angle were not done, the theory and the practice seemed to generally follow each other (e.g. a mass of sharp 'interference' lobes at the 50m ASL site). A similar effect was noticed while operating 2m EME from KH8 earlier this year from a site well elevated over the ocean (from memory, 60mH). No doubt there will be differences at 70m above 'ground' at your site, and there is only one way to find out ...

73,

[VK2FAK]

Hi all....

Yes it is an interesting question Steve.

a question, you say you have a 70m cliff , how near is it.....and are we talking the side of mountain situation were it just continues on at 70m below your QTH, maybe some low rolling hills ? I would think all these issues would mess up ground gain.......also...is it just rock...or is there plenty of bushes, trees and the like...would not think vegetation will play nice with any ground reflections

[VK2XV]

An article I wrote for NZART Break-In magazine in 2010 combines theory and practical results of 2m antennas at various heights ... including at 50m above the ocean.

Thanks Bob - most interesting. I wonder - can you remember if the observed effect at Cape Reinga went down to the 2 deg mark as shown on the theoretical graph ?

Another reference I have found for high antenna heights is this...

http://www.sydneyobservatory.com.au/201 ... ron-ekers/

...where they used a "Cliff Interferometer".

But as you say - over ground ????

[VK2XV]

G'day John,

a question, you say you have a 70m cliff , how near is it.....

As mentioned in previous posts it is at the back of the block here (3/4 acre) - so antenna will be at the edge. The edge drops away almost vertically (60 degree slope) 70m down to farmland with almost no trees for about 3km.

[ZL1RS]

... I wonder - can you remember if the observed effect at Cape Reinga went down to the 2 deg mark as shown on the theoretical graph ?

According to the times logged, the 2m EME stations were worked at the Cape with the moon at 12 degrees ... 11 degrees ... 8 ... 5.6 ... 4.6 ... 3.3 ... and 1.1 degrees. I normally log the end time of the contact, so the moon would be a degree or so higher 4 minutes earlier at the beginning. I also remember that during one or more contacts the moon 'went through a null' and the contact was completed on the 'upswing' of the next peak.

...But as you say - over ground ????

... and as I said "there is only one way to find out" ...

Portable EME is a lot of fun, take the Nike advert hint and 'just do it'. If you have not been on 2m EME, with a little "advertising" prior to the event you will have plenty of takers for a "new initial" ... and even more if the site is a "new square".

73,

[ZL3TY]

Steve

My QTH is on a 70m asl terrace about 1km from ocean. The antenna is about 10m high at about 20m from the edge of the terrace which drops away at about 45deg. I get great results over the sea on terrestrial and EME. On EME frequently decodes at down to -1deg.

Your picture looks pretty good to me, maybe a bigger antenna would help...

Bob ZL3TY

[VK2XV]

Hi All,

Thanks for all the responses. I think I have an answer - there is no theoretical answer.

As Bob says - I just have to do it.

As a final comment I reckon that for 70m height on 2M the angle between peaks is (from RA interferometry) is ~0.8 degrees. Therefore the first peak is ~0.4 above the horizon (first null is at horizon). The second peak is at ~1.2 degrees and so on in ~0.8 increments. The higher elevated peaks will come via ground reflections closer in, while the lower elevated peaks will come via reflections further out. My feeling about this is that the reflections further out over earth will be more diffuse and ground gain will drop (ground only comes from coherent constructive interference) for the lower elevated lobes. I would not be surprised that over earth the lowest couple of lobes (say 1st, 2nd, 3rd..) will actually have lower ground gain than for lobes a little higher (say 4th, 5th 6th). That is, I would say there is a peak in ground gain which occurs not in the first lobe but a lobe further up - followed by a decline again in higher elevated lobes.

Please note: the above comment is for 2M at a height of 70m.

Thanks & 73