This discussion serves to relate my experiences with quarter wave vertical antennas, both homebrew monobanders and a commercial multiband trap antenna, specifically the 5BTV as used in amateur radio.
Hopefully this information will be useful to the amateur radio community. My conclusions in regards to antenna performance are relevant to the RF characteristics of my QTH, and should not be regarded as conclusive in any way.
Wednesday, October 8, 2008
Tuesday, October 7, 2008
Installation 5BTV ground mounted with radials
About a year ago I decided to install my 5BTV in a ground mounted with radials configuration. I have used this antenna elevated with radials for Field Day, but wanted an easy to maintain "clean" installation without the space requirement for the spiderweb of radials when elevated.
I also did not want to spend any money on the installation, using only materials on hand. Wire for radials came primarily from non-metallic sheathed conduit scraps.
The largest open space in the yard with 24' then a fence on one side and 24' then a building on the other side was selected and the 4' support pipe (1.5" galvanized) pounded into the ground. Then PVC pipe was buried and RG8M coax feedline installed.
Conduit and RG8 would be better, but that was not on hand and would have cost money. Besides, I don't use an amplifier. A RF choke was wound around a plastic coffee container.
Then, I made a radial ring out of copper flex tubing and soldered short wires to it so I would not have to solder to the ring in the field.
Radial installation began. I decided on 40 24' radials based on the wire I was able to scrounge. Most of the radials are 12ga solid wire, some insulated, some bare. This homebrew device was used to cut grooves in the soil for the wires:
It was winter at the time so the ground was soft. I buried the radials about 1" down and soldered them to the radial ring. Hard part completed! Next the antenna was mounted and guy wires installed.
Tuning was next, which was straightforward except for having to adjust the 20M trap (the clamp the instructions say to DO NOT LOOSEN) as the tubing between the 15M and 20M traps could be not be shortened enough.
I also polished up the antenna and sprayed it with clear coat for preservation.
About three months later, I got ahold of some more wire and added another 20 radials. This time, I did not cut grooves but instead tacked the radials down with bits of wire expecting them to disappear in the weeds eventually.
Finally, for field day I added 64 temporary radials made out of telephone wire, bringing up the count to 124 radials, all around 24' long give or take a couple feet. After FD, the telephone wire radials were removed.
I also did not want to spend any money on the installation, using only materials on hand. Wire for radials came primarily from non-metallic sheathed conduit scraps.
The largest open space in the yard with 24' then a fence on one side and 24' then a building on the other side was selected and the 4' support pipe (1.5" galvanized) pounded into the ground. Then PVC pipe was buried and RG8M coax feedline installed.
Conduit and RG8 would be better, but that was not on hand and would have cost money. Besides, I don't use an amplifier. A RF choke was wound around a plastic coffee container.
Then, I made a radial ring out of copper flex tubing and soldered short wires to it so I would not have to solder to the ring in the field.
Radial installation began. I decided on 40 24' radials based on the wire I was able to scrounge. Most of the radials are 12ga solid wire, some insulated, some bare. This homebrew device was used to cut grooves in the soil for the wires:
It was winter at the time so the ground was soft. I buried the radials about 1" down and soldered them to the radial ring. Hard part completed! Next the antenna was mounted and guy wires installed.
Tuning was next, which was straightforward except for having to adjust the 20M trap (the clamp the instructions say to DO NOT LOOSEN) as the tubing between the 15M and 20M traps could be not be shortened enough.
I also polished up the antenna and sprayed it with clear coat for preservation.
About three months later, I got ahold of some more wire and added another 20 radials. This time, I did not cut grooves but instead tacked the radials down with bits of wire expecting them to disappear in the weeds eventually.
Finally, for field day I added 64 temporary radials made out of telephone wire, bringing up the count to 124 radials, all around 24' long give or take a couple feet. After FD, the telephone wire radials were removed.
Monday, October 6, 2008
Performance
First impressions of the antenna were favorable. Overall, the 5BTV outperformed my former windom antenna on all bands. However, the windom was never installed at optimum height and was very close to trees.
The most useful performance test was last field day, when I put up a homebrew 20M monoband vertical with 8 radials (antenna base 45' high) and a homebrew 40-10 trap dipole mounted as a sloper with the highest point at 35'.
Consistently, the 5BTV was outperformed by the monobander and dipole. On 20 and 40, the 5BTV exhibited 2 or 3 s-units weaker reception than the higher antennas. Weak stations were often heard by the monobander or dipole, but not with the 5BTV.
Here are the SWR curves after installation with 40 radials:
The very narrow bandwidth on 80 was expected, and indicates that the antenna is working. However, the relatively high SWR on 40 was a surprise, although my antenna tuner could handle it. I contacted technical support and was advised to install a coil between the radiating element and ground as shown:
This lowered the SWR at 7.2Mhz to 1.5:1, with the curve following along. 80M reception was slightly degraded after the installation of the coil, and AM broadcast reception was significantly degraded. As I enjoy using my HF rig to listen to AM broadcast, I decided to reject the coil "fix" and let the antenna tuner deal with 40M.
The SWR curves after increasing to 60 radials and then to 124 radials did not vary significantly. The slight variances may be caused by seasonal changes and moisture content of the soil.
The most useful performance test was last field day, when I put up a homebrew 20M monoband vertical with 8 radials (antenna base 45' high) and a homebrew 40-10 trap dipole mounted as a sloper with the highest point at 35'.
Consistently, the 5BTV was outperformed by the monobander and dipole. On 20 and 40, the 5BTV exhibited 2 or 3 s-units weaker reception than the higher antennas. Weak stations were often heard by the monobander or dipole, but not with the 5BTV.
Here are the SWR curves after installation with 40 radials:
The very narrow bandwidth on 80 was expected, and indicates that the antenna is working. However, the relatively high SWR on 40 was a surprise, although my antenna tuner could handle it. I contacted technical support and was advised to install a coil between the radiating element and ground as shown:
This lowered the SWR at 7.2Mhz to 1.5:1, with the curve following along. 80M reception was slightly degraded after the installation of the coil, and AM broadcast reception was significantly degraded. As I enjoy using my HF rig to listen to AM broadcast, I decided to reject the coil "fix" and let the antenna tuner deal with 40M.
The SWR curves after increasing to 60 radials and then to 124 radials did not vary significantly. The slight variances may be caused by seasonal changes and moisture content of the soil.
Sunday, October 5, 2008
Conclusions
A quarter wave vertical mounted 1/2 wavelength or higher with at least four radials will outperform a ground mounted radial even with 120+ radials. The quantity of radials does not substitute for height.
I noticed a slight gain when increasing the radials from 40 to 60, but no improvement going from 60 to 124.
Any antenna is always better than no antenna, and the ground mounted installation is very unobtrusive and easy to disguise as a flagpole in restricted neighborhoods, and overall has satisfactory performance.
Again, my conclusions are based on the characteristics of my QTH. A trap vertical with 120 60' radials and 100' clear space around the antenna might have a quite different result.
The brand name of the 5btv antenna has been intentionally omitted, as I am getting comments from people looking for an entirely different subject.
I noticed a slight gain when increasing the radials from 40 to 60, but no improvement going from 60 to 124.
Any antenna is always better than no antenna, and the ground mounted installation is very unobtrusive and easy to disguise as a flagpole in restricted neighborhoods, and overall has satisfactory performance.
Again, my conclusions are based on the characteristics of my QTH. A trap vertical with 120 60' radials and 100' clear space around the antenna might have a quite different result.
The brand name of the 5btv antenna has been intentionally omitted, as I am getting comments from people looking for an entirely different subject.
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