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75M Local Dipole Optimization - real whirl tests




 
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Author Topic: 75M Local Dipole Optimization - real whirl tests  (Read 4884 times)
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K1JJ
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« on: May 05, 2013, 05:56:54 PM »

I'd like to post some results I've found when determining the best height and configuration for LOCAL work using a standard 75M 1/2 wave dipole fed with 7/8" hard-line.   (Open wire feed would be the same)

I have a dipole fed with hard-line up at 60' in the center with the ends at about 50'. It works like a bomb for local contacts between 0-300 miles on 75M.  When compared against my SW and NE wire arrays at 190' the lower dipole is usually  10-15dB stronger locally, as to be expected.  So it's working as it should.

The problem with this dipole is it is too close to the house and picks up computer and power line noise, so I wanted to relocate it about 300' away at the same height and orientation.  I put up a second, temporary  dipole at 48' as a low inv vee, also fed with hard-line. (located 300' away, same orientation)  One end was at 30' and the other end at 15' - obviously a lower antenna.  I wanted to make SURE that the new dipole worked as well the the old, so ran some tests as I raised its height.

The initial differences were remarkable...  Doing instant receive A/B tests on locals, the lower dipole was down between 6-8 dB almost consistently.  This is quite a lot considering the height, but the inv vee angle was sharp. Difference probably due to a combination of ground loss and higher take-off angle of the low dipole.

I then raised both ends of the new dipole to a flat top, all at 48'.  The lower dipole was now down only about 3dB from the old dipole at 60'.  BTW, the old reference dipole is over the house, so is not in the clear.  The new dipole at 48' is in the clear.

I then raised the new dipole to 60' at the apex, 50' on one end and 60' on the other end. Almost a perfect flat top and also straight. Both dipoles now at about the same height.  The new dipole now was usually as good and sometimes a dB or so louder than the old house dipole.  The difference in noise was like night and day. The new dipole is so quiet at first I thought the connection was bad.

I believe there is a very important thing to be learned here.  Whether we are dealing with ground losses, lower angles of radiation or whatever, there seems to be a sweet spot for 75M LOCAL work that wants a dipole to be flat and near to 1/4 wave above ground.  I see it time and again where a station will be hard to copy and tells us his dipole is 30-35' high with the ends near the ground. But later they raise it up to 60-70' and the difference is more than we'd expect.. it is quite dramatic at times.  They join the big guns and are consistently loud.

Steve/ HX and I have posted antenna modeling data that supports these ground loss numbers, but I wanted to see for myself in the real whirl, adding in real propagation, real incoming radiation angles.  Indeed, a dipole in the clear that is an inv vee, probably performs as its AVERAGE height above ground.  In this case when the new dipole was at 48' apex and one end at 15', the average height was about 30'.  It certainly paid the price.

We can't always get a dipole as high as we wish, but it's well worth the effort to try. A 6-8 dB antenna deficit is never recovered on our receiving end and would take a linear amp to make up on transmitting.

T
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« Reply #1 on: May 05, 2013, 07:44:00 PM »

T tnx for the information.  Yep I decided a few years ago that the Inverted V near ground is, as they say in Australia, a mug's game.  The old information about them in handbooks, promoting them because of the 50 ohm feedpoint Z, and supposedly partial vertical polarization, led a lot of hams astray in my opinion. 

I can't get the center fed dipole here up to 60 feet but I was pleased that your 48 foot flat top was 3 dB down from 60 feet (however that's a doubling of power).  Brandon K5IIA's signal improved dramatically when he got his dipole up to 70 feet.   One thing I've wondered is if the ends of the flat top dangle straight down as mine do, but are perpendicular to the ground surface with the ends up about 20 feet or so, how much loss is there i.e. is it similar to an inverted V's ends in that regard.

Rob
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flintstone mop
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« Reply #2 on: May 05, 2013, 08:36:13 PM »

TNX Tom
You gave a nice study about 40M a few years ago with your stacks of Yagis. And how things can change dramatically throughout the day. High angle low angle reception.
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Fred KC4MOP
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« Reply #3 on: May 06, 2013, 04:08:41 AM »

Thanks for the report on your antenna work, Tom. Well done. It reminded me of this one which is also a good read.
http://amfone.net/ECSound/K1JJ21.htm

-Don
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K1JJ
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« Reply #4 on: May 06, 2013, 12:23:14 PM »

Thanks for the comments, guys.

I guess we are preaching to the choir in a way, about something we already know. But I think many of us still thought that an inverted vee with a sharp angle still worked almost as well (or better due to vertical component) than a dipole. But maybe not for local high angle work.  Perhaps if the apex is at 80' and the ends were at  30' the average height would be moved up and work much better for local.

Another observation: For the last year I had up a second "local" dipole at 100', flat top. It was usually down about 10dB from the local dipole at 60'.  It showed me that there is a certain height that is optimum for local work and as we go higher, the take-off angles start to get too low. This exact height will vary depending on our own QTH RF ground depth, topography, etc.  We all need to experiment and find the best height, if possible.


Rob, I have not done any modeling using the dipole's ends dropping straight down, though I know a few guys using it and they appear to have signals that are par with flat dipoles.  Bob / KBW uses this technique and is one of the loudest locals in here.

T

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KK4YY
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« Reply #5 on: May 06, 2013, 03:32:26 PM »

For local work a 10dB drop sounds like a lot going from 60' to 100'. I suspect some of that can be attributed to the higher dipole having better azimuthal directivity. If this is the case the 10dB figure may not play out in all directions. There may also be a null developing at some vertical angle on the higher dipole that the some "locals" fall in to. Complicated. For me, it's easier to imagine the very low dipole pattern... a balloon. It gets all 3D ish when we go higher - hard to think that way. Better directivity, lower takeoff angle, nulls develop. Alas, nothing is simple. The best bet is to have more than one antenna as there is no one best antenna. But, given our limitations, we often have to make a choice... a compromise.

The 75M dipole at 60' or so looks like a good compromise for local work. It still has a fairly omnidirectional pattern, like a very low dipole, but has less ground loss. Above that, ground loss would probably be a small factor but pattern development starts to become significant. Not authoritative -  just my opinion.

Your "real whirl" tests are very valuable and may save someone the trouble of raising an antenna to 100' only to be disappointed with the results. KUTGW.

-Don
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K1JJ
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« Reply #6 on: May 06, 2013, 04:51:53 PM »

Hi Don,

I know some guys with dipoles at 90' who do very well locally.  Steve / QIX has his at 90' and also Joe/ IWQ.  They both have rock crushing local signals.  So that's why I was surprised when the dipole at 100' was so far down.  I compared it against the same 60' high dipole as in the last test. Used a rapid A/B switch, which is the best way to do comparative antenna tests.  I even thought I had a feedline problem, but it tested out OK. It's 7/8" hard-line buried in a 4" PVC pipe underground..

For a few months I used just the dipole at 100' and noticed locally my reports were down compared to some of the local strappers.   When I did the A/B between the two, it was all clear. I then took down the 100' high dipole.

So... as we progress and find even this 60' dipole has bad noise from the house AND I was getting into the security system being the dipole was right above the house... too close.  The dipole at 60' mounted 300' from the house is a keeper.

It's an evolutionary thing to find the best antenna for the job.  No doubt that two antennas, one for local 0-300 miles and one for 300 and beyond would work well on 75M.  We just need to find what fits at our QTH.


T
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« Reply #7 on: May 06, 2013, 07:03:22 PM »

Tom,
how does ground conductivity play with various heights?  The reason I ask my dipole is suspended over wetlands yet it's probably about 45 to 50ft at the center. I get good reports even at a 20w level on 75m. It's also not straight. It's folded horizontally at about 120 or so degrees.  I call it the lazy inverted V.
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« Reply #8 on: May 06, 2013, 07:38:05 PM »


Rob, I have not done any modeling using the dipole's ends dropping straight down, though I know a few guys using it and they appear to have signals that are par with flat dipoles.  Bob / KBW uses this technique and is one of the loudest locals in here.


okay thanks Tom
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Steve - K4HX
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« Reply #9 on: May 06, 2013, 09:09:18 PM »

Modeling shows a slight difference, less than a dB. The plot compares at flat-top dipole at 60 feet in red and a bent dipole (center section at 60 feet and 30 foot long vertical sections off each end) in black dashed. So the ends are at 30 feet above the ground.

My guess is that the loss will be a bit more in some real world situations since the vertical sections couple to other metallic objects, house wires, etc. The modeling is done over real ground but no other conductors are involved.


* 80mdipflattopvsbent30.png (140.47 KB, 600x579 - viewed 375 times.)
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K1JJ
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« Reply #10 on: May 06, 2013, 09:27:04 PM »

Tom,
how does ground conductivity play with various heights?  The reason I ask my dipole is suspended over wetlands yet it's probably about 45 to 50ft at the center. I get good reports even at a 20w level on 75m. It's also not straight. It's folded horizontally at about 120 or so degrees.  I call it the lazy inverted V.

Cool on the dropped ends being within a dB, Steve. Gee, that's as close to a free lunch as it gets.


Bob, I imagine that the better the RF ground, the higher the take-off angle.  But the ground losses are less too.  The wetlands may give you a killer local signal.  I know that Steve / HX's location is similar and he takes out most stations in his area into Eu on both 75M and 40M. He can hear extremely well and has guys axing him often what he is using for RX.  I think wetlands do help all around.

Over here I have very poor rock ledge Earth, so my RF depth on 75M may be as much as 15'- 20' deep. So, a dipole at 100' might look like it is at 120', producing very little high angle. Maybe that's why my  dipole at 100' didn't play well for 75M locals compared to the dipole at 60'.


As we know, comparative antenna measurements need to be made with the two antennas at least a wavelength apart, ( or more, to prevent interaction) need to be in the same orientation and need to be compared with a fast A/B switching.   To raise or lower a single antenna expecting to tell a difference by signal reports on the air is too slow and subjective - and prone to fast changing condition errors.

T

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« Reply #11 on: May 06, 2013, 09:37:15 PM »

I took one of my dipoles down from the phased array. I will take the other dipole down when I get a little closer to hurricane season. I do notice right off the bat that all the stations up the east coast are way down. I think they would hear me but nothing like with both dipoles. So right now I am using a single dipole at 75 feet or maybe a little higher flat top. I wont say I see a huge difference with close in stations but they are up some. The big difference is around 3.875. May just be spring time conditions causing some attenuation also.

The only rock on this land is stuff I bought for the drive way.
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