| Author |
Message |
    
Marconi
| | Posted on Tuesday, September 03, 2002 - 5:05 pm: | 
|
In ARRL Antenna Handbook under the topic, Ground-Plane Antennas, there is mentioned the following;
".... It is obvious that with 1/4 wave radials the antenna and any one radial have a total length of 1/2 wavelength and therefore will be a resonant system. However, with only one radial the directive pattern would be that of a half-wave antenna bent into a right angle at the center; if one section is vertical and the other horizontal this would result in equal components of horizontal and vertical ploarization and a nonuniform pattern in the horizontal plane."
If these statements are true, then I assume that one fixes this problem by adding a balanced ground plane around the base of the vertical when adding 3, 4, or more ground radials. In other words the nonuniform pattern problem will be more or less fixed as to the pattern it produces.
My question is, what happens to the equal components of horizontal and vertical polarization when you add the other elements? Does the antenna still exhibit equal portions of both horizontal and vertical polarization?
What happens to the polarization if you move the radials more into the vertical plane as in the Starduster antenna? Do we loose some of the horizontal polarization and gain some vertical?
Marconi |
Tech833
| | Posted on Wednesday, September 04, 2002 - 1:48 am: |
|
Yes, no, sorta.
In a counterpoise system you do not have any 'horizontal polarization' like you are thinking (except for magnetic field). A center fed half wave dipole has an impedance of around 70 ohms. If you 'bend' the counterpoise up toward the vertical element, the impedance drops. When you reach around 45 degrees from vertical (sorta like the 'Starduster'), the impedance is around 50 ohms.
Now, if you bring the counterpoise to a 90 degree angle from the vertical element, the impedance is around 36 ohms. You make up for that by slightly lengthening the vertical element and taking the additional reactance that results. The traditional 1/4 wave you see with completely horizontal elements is more for convenience than anything else.
The counterpoise system does not radiate much at all, as there is almost no current anywhere on it. Also, you keep looking at the counterpoise as 4 separate elements (ground planes) when in fact, once the elements are spread around the base, it acts like one solid mass (like the reflector of a welded tube grid dish does). Your antenna 'sees' the 4 ground planes as a semi solid metal square under the radiator, like the roof of a car. |
Marconi
| | Posted on Thursday, September 05, 2002 - 2:35 pm: |
|
833, I cannot disagree with anything you say here. I have been trying to understand and explain a rather remarkable observation I made some years ago using a new horizontal 4 element yagi that I had just installed.
I had two buddies that lived right next door to each other about 4 miles from my place. Their antennas were a little less than 60' apart. Windy had a Sigma IV up about 35' to the base and Johnny had a Starduster up about 60' to the hub. With my Kenwood TS50 connected to my A99 they both gave me the same exact signal, 20/S9. When I switched to my beam I could still hear the SD'r but it showed no signal. The signal from the Sigma was reduced to an S5 signal. Of course I was amazed as I did not expect that much response considering I was pointing directly at them. Both stations said they were seeing about the same thing from their end. We fussed and discussed this thing for a long time.
Later I made some comparisons with a number of other vertical type ground planes in the area, including two other SD'rs. One SD'r was only about 5 blocks away and the other was about 30 miles away. The station 30 miles away had a SD'r and a Maco V-5/8 in the top of two pine trees at just over 100'. He said his Maco was his best antenna. With my A99, the Maco did show a two S-unit better signal on his end at about S9. I saw a one S-unit better signal from his Maco at my end. However, just like in the comparisons with the neighbors, I got similar results with both his antennas.
The station that was close really flooded me but the beam still showed a big reduction in his signal even at that close distance.
In all my comparison checks only the SD'r reacted as described to the horizontal beam. We finally concluded that the old SD'r was just a lot less sensitive to horizontal signals. Is the old SD'r just that poor at doing its job or is there something to the polarity issue I raise?
Marconi |
Tech833
| | Posted on Friday, September 06, 2002 - 10:10 am: |
|
Polarity does not remain stable as it leaves the antenna. You are also making the assumption that the magnetic field and the electrical fields are remaining exactly in phase.
The Starduster is a half wave dipole with the counterpoise raised slightly to give the feedpoint a 50 ohms value. Don't look at the ground planes as individual elements, look at them as a solid 'cone' around the base of the antenna. They serve to not only balance the current distribution from the feedpoint, they also decouple the RF from the feedline.
You are also forgetting that a portion of the Maco's radiating element is horizontal in the ring. If the ring were slightly bigger and the vertical element were shorter, you would have a 'ring-stub' antenna. See http://www.bext.com/antennas/telecom/tfc1k.htm
I just designed a set of these for an FM in California. By playing with the size of the ring and the vertical elements, you control the percentage of vertical to horizontal polarization.
Does this help? |
Hoosiercardinal
| | Posted on Friday, September 13, 2002 - 6:34 pm: |
|
Although wayyyyy above my head this is very interesting!! |
|
