6M Angle of Takeoff
or: Is My Antenna Too High?
When most hams talk about antenna directivity, they are referring to the East/West, North/South sense.
The other part of directivity is what I want to discuss today, the "vertical angle of takeoff", or just "angle of takeoff".This is the angle that the main lobes leave the antenna with respect to ground. It is not "vertical", that just refers to the aspect from which it is viewed> a vertical slice of space centered on the antenna axis.
The location
The Black Hole:
This station is located in EM49, near the edge of what we call the black hole. Stations as close as 50 miles to the west and to the north do not get the propagation we can get here to Europe and Africa. Stations a few hundred miles to our west may have worked DXCC, without ever having heard Europe (N0LL). The Black Hole is essentially the upper middle 1/3 of the USA. Propagation on 6M is difficult at best. W7XU told me that he goes on 6M DXpeditions because he likes to work DX and can't do it from home ( and with a world class station to boot!).
Hams on the western edge of the Black Hole all the way out to the west coast get propagation to the Oceanic region that is rare from the east coast.
The tools used for the examination:
First thanks to KB0PE, K0CBA, K0TP, WB9CTN, KB9CTN, N0PB for work on the systems.
Mike W6MYC made the H frames and support struts, rotors and elevation rotors and some of the 6M beams ( 11 el and 7 el)
W1JR designed "Boomers" (Cushcraft).
K5GW handled the phasing lines.
K0RL designed and made the power splitters.
K0AZ helped with tower and feedline acquisition.
Antenna 1: M2 11 element 51 foot boom on a Rohn 100' SSV fed with 1 5/8 Heliax (OMG)
Antenna 2: M2 7 element on a 56 foot crankup fed with 1/2" Heliax. Height adjustable 23-56 feet.
Antenna 3: Four 6M 6 element Boomers, H frame 25' square, 72 foot crankup. Height adjustable from 45 to 85 feet center of H frame. Azimuth plus full elevation control.
The examination technique:
At first all we had was Es opening of various types. Logic dictated a single radio and an antenna switch to do A-B-C comparisons of the different antennas at different heights and elevations. Very quickly the folly of this technique was demonstrated. band conditions, QSB, Faraday rotation etc. just simply changed too fast for the individual reading to have any meaning!
Next was tried three different rigs each on a different antennas. Now the failure was in the operator. Too much going on at once to make any sense of it!
The final setup was elegantly simple. Two radios, and a switching arrangement that gave me access to any two antenna at once. This was fascinating to watch work. Since the two farthest apart antennas were about 500 feet distance with the 7 element in the middle nearest to the shack, propagation moving across that space could easily be noted!
Next realize that the examination was all about skywave. On groundwave, the long, high antenna was always best. Makes sense. Line of sight and all that.
On skyways, Es, it was a vastly different picture. In no case of long haul propagation by Es did the long, high antenna (system #1) outperform the other two systems when they were located at a lower height. This includes every observed path of Es, normal distances, Midwest US to Europe multi hop, and Es linked TEP to South America ( all the way to VP8). All different heights were tried, over very many openings for the whole sunspot cycle. Some pretty remarkable and surprising contacts resulted.
One notable example was a lengthy opening to LU, Argentina. The station was a good operator, had a strong enough signal and was calling CQ. And calling CQ again and again with no takers. I was calling him on the high 11 element antenna with my usual 40 Watts. No response. Called him again with 400 Watts, no response. Then switched to my lowest antenna can called at 40 Watts, got an instant response and 5X9. During the QSO I switched back to the tall antenna, and nada.
This has been observed many times against local DX'ers who have only one 6M antenna, and mounted VERY high. Time after time, I could skunk them with a low antenna and low power. A pattern was developing.
For the next several years I spent my time listening and comparing. The elevation capability was experimented with and the results were that in almost every circumstance, the signals were best with a 10 degree to 15 degree elevation. Sometimes MUCH better.
Now compare a single antenna at different heights knowing that a 10 degree angle might be optimum:
Download the photo files of two different 6M antennas at various heights here:
http://www.qsl.net/k0ff/Angle%20of%20Takeoff/Animation%20Files
Now load the files into a picture reader program such as XN View. Use the "next" button to scan through the pictures at about 1 per second. The resulting animation will demonstrate the effect of raising the antenna from 20 feet to as much as 110 feet. Keep in mind that 10 degree "magic Es angle". Not only do the lower antenna favor this angle, the taller ones place a deep null at that point.
Viola!
During all these observations it became clear that European propagation from here followed very simple rules. If I could hear W3's, forget Europe. The skip was too short to make it. If K1LPS or VE1YX were the only stations being heard here (a long hop), then when they faded out ( seemingly dead band conditions, no signals from out of local area), that's when I started trolling for Europe.
It did not work EVERY time, but every time it DID work, it followed that pattern.
Take 23 June 1998, the above conditions were met, and I called CQ with the antennas tilted up to 12 degrees. No signals at all on the band. Answered by S59A out of the blue with 578 sigs. Followed by 9A8A, DL's, OK's SP's. Nice day's work.
On 23 June 1999 the elevatable antenna was once again set to about 12 degrees up angle, and the above conditions were met. After one CQ I was immediately answered by EH8BPX, followed by a run of S59A, more EH's, LX, OK's, Gi's, As matter of fact the propagation was following the "grey line" at their end. W3EP called me afterward to report that no signs were evident al all on the east coast, they heard nothing!
F2 at last!
Finally in Nov 2001 we got F2 on 6M here! The biggest opening I have heard since 1981. Don't forget that in '81's wild openings, most Europeans did not have 6M privileges. We had to work England and Germany on 6M and listen to them on 10M, such a contact did not count for DXCC.
Now it was our time to clean up, and to make it short, the long tall antenna was king! Bigtime KING, outperforming every other antenna on the order of 6 S-Units, and proving to me that the angle of takeoff theory is right on. Finishing my DXCC was easy although long awaited ( 36 years at that time).
Recommendations
If you happen to live near any coast of the USA, my recommendations are simple: put up an antenna and turn your radio on. DX will happen. K2RTH in Homestead Florida made amazing DX contacts with his 28 foot tall antenna!
If on the other hand, your location is less favorable, a little more finesse may be in order.
The natural ham tendency of putting antennas as high as physically and financially possible may not always be the best solution. Antenna height should be thought of in terms of wavelength, not feet.
It is doubtful that you will ever be able to put a 160 or 80 meter antenna up 2 times 5/8= 1.25 wavelengths. On 6 meters, this is easy and the best place to start (25 feet = 12 degrees, ideal for Es.).
Double that to 2.5 wavelength, 50 feet, cut the angle of takeoff in half to 6 degrees (good for local, tropo, F2) and double that again for a 100 foot height, 3 degree takeoff and a killer local, tropo and F2 antenna.
Two or three antennas, each for a different use.
In general it is better to have a single long boom antenna than to try to stack two shorter boom antennas.
If your location is prone to local QRM from other hams ( nets, ragchewers etc), consider mounting the low antenna in the vertical polarization and tilting it up 10 degrees.
Local horizontally polarized hams will be attenuated up to 20 dB with virtually no change in the skywave signals.
If QRN is a problem don't hesitate to contact the power company. QRN costs them revenue money! My power company was very cooperative, to the point of buying a QRN locator, after observing mine in action. After a bit of work by myself and their crews, I am now able to operate 6M and all bands without the need of a noise blanker. Nice.
Keep coax losses as low as possible, shoot for 1 dB on all bands.
It's been said that preamps are not needed on 6 meters. I don't use one but have tried them and they do help in some instances. Mast mounted preamps are the way to go, but can be problematic in the heat of battle.........errrrrrrrrrrrr....I mean while making a DX contact.
See you in the pileups.
Geo>KØFF
WØSIX 6M Club.