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Radials
On Friday, April 4, 2014 9:16:01 PM UTC-5, Fred McKenzie wrote:
3. Ground is relatively flat. Drooping radials to approximate a sleeve dipole is stretching the definition of a ground plane! The best modeled version of the 5/8 with 5/8 radials scheme, I consider more of a dual 5/8 collinear than a 5/8 ground plane. You don't get the full 5.1 dbi free space gain of the straight collinear, but you get fairly close. But.. I don't really care about the name.. Just as long as they work. :) |
Radials
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Radials
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Radials
In article ,
Ian Jackson wrote: I understand that my 2m 5/8 mobile antenna (on a magmount) is essentially electrically a 6/8 (ie a 3/4 wave - hence a good match). The actual whip is around 5/8, and the other 1/8 is the 3-turn spring steel 'loading' coil at the bottom end. Again my understanding is that a 5/8 gives the maximum broadside gain (a tiddly bit more oomph than a 1/2 wave), and if you make the antenna longer, the predominant broadside lobe collapses, and most of radiation moves to the higher angle lobe. I read somewhere that the maximum gain toward the horizon, was obtained with a 0.58 wavelength vertical element rather than 5/8 (0.625). The advantage of a 5/8 antenna with a flat ground plane, is that its impedance is capacitive with a 50 Ohm resistive component. A small inductor (loading coil) cancels the capacitance. Fred K4DII |
Radials
On Saturday, April 5, 2014 8:40:31 AM UTC-5, Fred McKenzie wrote:
In article , Ian Jackson wrote: I understand that my 2m 5/8 mobile antenna (on a magmount) is essentially electrically a 6/8 (ie a 3/4 wave - hence a good match). The actual whip is around 5/8, and the other 1/8 is the 3-turn spring steel 'loading' coil at the bottom end. Again my understanding is that a 5/8 gives the maximum broadside gain (a tiddly bit more oomph than a 1/2 wave), and if you make the antenna longer, the predominant broadside lobe collapses, and most of radiation moves to the higher angle lobe. I read somewhere that the maximum gain toward the horizon, was obtained with a 0.58 wavelength vertical element rather than 5/8 (0.625). I'm not sure if that gives the maximum gain vs a .64 wave, but it gives the cleanest pattern, with the least radiation skewing upwards. So most of the AM broadcasters that use 5/8 radiators prefer them on the shorter side from what I've read. The advantage of a 5/8 antenna with a flat ground plane, is that its impedance is capacitive with a 50 Ohm resistive component. A small inductor (loading coil) cancels the capacitance. I didn't see much if any changes in the loading coil no matter what I did for radials. IE: I used the same coil for all of them, and never had to change it when I added longer radials, etc. I did not use a grounded coil. I just ran it in series. That way I was able to use the antenna on 30m as a 1/4 wave. And the same when I had the full size 40m 1/4 GP. I also used it on 17m as a 5/8 GP, and used a 24v relay to bypass the coil for 40m. That way I could switch from the shack. Kinda hard to reach when the base of the GP is 36 ft up.. lol.. I used a full size 32 ft aluminum radiator, with the tip made of car whip to reduce weight and wind load. The lower tubing was double walled to be stronger. Almost 70 ft to the top of that thing. It kicked butt on 40 and 17 both. :) Good on 40m DX late at night, and I was running a KW+ on top of that, just to make sure I browned the food across the ponds. I also had a 40m horizontal dipole at 36 ft, and to DX, the vertical was usually about 2 S units better to the U.S. coasts, and often 4 S units better across the pond. The longer the path, the better the vertical did. Also had a good ground wave, which was often noticed in the daytime. |
Radials
El 05-04-14 9:46, Ian Jackson escribió:
In message , writes On Friday, April 4, 2014 9:16:01 PM UTC-5, Fred McKenzie wrote: 3. Ground is relatively flat. Drooping radials to approximate a sleeve dipole is stretching the definition of a ground plane! The best modeled version of the 5/8 with 5/8 radials scheme, I consider more of a dual 5/8 collinear than a 5/8 ground plane. You don't get the full 5.1 dbi free space gain of the straight collinear, but you get fairly close. But.. I don't really care about the name.. Just as long as they work. :) I understand that my 2m 5/8 mobile antenna (on a magmount) is essentially electrically a 6/8 (ie a 3/4 wave - hence a good match). The actual whip is around 5/8, and the other 1/8 is the 3-turn spring steel 'loading' coil at the bottom end. Again my understanding is that a 5/8 gives the maximum broadside gain (a tiddly bit more oomph than a 1/2 wave), and if you make the antenna longer, the predominant broadside lobe collapses, and most of radiation moves to the higher angle lobe. The "theoretical" more gain for a 5/8 lambda radiator over a half wave dipole is only valid when towards the direction of reception, you have a fully constructive image antenna in the ground. To fullfill this: 1# The elevation angle needs to be well above the (pseudo) brewster angle, to make sure that the ground reflection is strong and more or less in phase. 2# The ground plane needs to cover at least about the first Fresnel zone as seen from the negative image antenna towards the reciever. When you look to real far field patterns of 5/8 lambda antennas as used in AM broadcast over land, the pattern only matches the theoretical pattern for elevation angle say above 5..10 degr (depending on soil type). For lower elevation, you are below the brewster angle and then the effect of the negative image becomes more destructive with decreasing elevation, hence decreasing gain. For HF, VHF and higher, the ground plane will never meet 2#. To meet this, the ground plane (metallic plane) needs to extend from the transmitter towards the receiver. You can't count mother earth as constructive ground as that serves as a destructive image as the elevation angle is well below the (pseudo) Brewster angle. For mobile LOS use, the advantage of the 5/8 lambda is not in the gain, but in its heigth, as propagation in a mobile path is roughly spoken proportional to h^2. So when the current center goes from 2.5 m (quarter wave on a car, 2 m band) to 3.3 m (5/8 lambda on same car), you win 2.4 dB. For a base station where the ground provision (for example 4 quarter wave radials) is already many lambda above mother earth, the additional gain due to the increased length is mininmal. -- Wim PA3DJS Please remove abc first in case of PM |
Radials
On 4/7/2014 8:54 AM, Wimpie wrote:
El 05-04-14 9:46, Ian Jackson escribió: In message , writes On Friday, April 4, 2014 9:16:01 PM UTC-5, Fred McKenzie wrote: 3. Ground is relatively flat. Drooping radials to approximate a sleeve dipole is stretching the definition of a ground plane! The best modeled version of the 5/8 with 5/8 radials scheme, I consider more of a dual 5/8 collinear than a 5/8 ground plane. You don't get the full 5.1 dbi free space gain of the straight collinear, but you get fairly close. But.. I don't really care about the name.. Just as long as they work. :) I understand that my 2m 5/8 mobile antenna (on a magmount) is essentially electrically a 6/8 (ie a 3/4 wave - hence a good match). The actual whip is around 5/8, and the other 1/8 is the 3-turn spring steel 'loading' coil at the bottom end. Again my understanding is that a 5/8 gives the maximum broadside gain (a tiddly bit more oomph than a 1/2 wave), and if you make the antenna longer, the predominant broadside lobe collapses, and most of radiation moves to the higher angle lobe. The "theoretical" more gain for a 5/8 lambda radiator over a half wave dipole is only valid when towards the direction of reception, you have a fully constructive image antenna in the ground. To fullfill this: 1# The elevation angle needs to be well above the (pseudo) brewster angle, to make sure that the ground reflection is strong and more or less in phase. 2# The ground plane needs to cover at least about the first Fresnel zone as seen from the negative image antenna towards the reciever. When you look to real far field patterns of 5/8 lambda antennas as used in AM broadcast over land, the pattern only matches the theoretical pattern for elevation angle say above 5..10 degr (depending on soil type). For lower elevation, you are below the brewster angle and then the effect of the negative image becomes more destructive with decreasing elevation, hence decreasing gain. For HF, VHF and higher, the ground plane will never meet 2#. To meet this, the ground plane (metallic plane) needs to extend from the transmitter towards the receiver. You can't count mother earth as constructive ground as that serves as a destructive image as the elevation angle is well below the (pseudo) Brewster angle. For mobile LOS use, the advantage of the 5/8 lambda is not in the gain, but in its heigth, as propagation in a mobile path is roughly spoken proportional to h^2. So when the current center goes from 2.5 m (quarter wave on a car, 2 m band) to 3.3 m (5/8 lambda on same car), you win 2.4 dB. For a base station where the ground provision (for example 4 quarter wave radials) is already many lambda above mother earth, the additional gain due to the increased length is mininmal. Excellent information, Wim. Thanks. John, KD5YI |
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