|Installing Vertical HF Antenna Radials|
|Articles - Technical|
|Written by Bryce Salmi|
|Friday, 04 September 2009 06:33|
Early in the Summer of 2009 I decided to install a ground radial system for the Hustler 5BTV antenna. This article is the result of nearly a month of work and well over 48 hours of sweat and blood invested. I'm not joking about the blood either, the mosquitoes hunted me in swarms. The culmination of the energy put into this project turned out to be well rewarded; I obtained an antenna of much greater performance in the end. My hope is that after reading this article you will have a better understanding of why a vertical antenna needs a ground radial system and how you may be able to apply my experience to your own antenna system.
Vertical antennas provide several benefits over horizontal wire antennas such as their ease of support and low radiation “take-off” angle great for DX (distant station) operation. When installing supports for a horizontal wire antenna, often spanning over 130 feet long is not a feasable option, a vertical antenna is often the way to go. Presented with a good radio frequency (RF) ground, a vertical antenna will exhibit a low take-off angle in all directions. The lower the RF is radiated closer to zero degrees above the horizon which means the longer a single “hop” will be allowing for really long paths bringing in distant contacts. A good RF ground will also make the antenna more efficient at radiating the power sent to it rather than heating the soil. Ground mounting a vertical antenna is a case where a good RF ground is essential for the performance of the antenna.
Here was my problem, bright as day. For several years I had a Hustler 5BTV (5 Band Trapped Vertical) antenna mounted in my yard to a four foot metal rod stuck into the ground. It was never used much, just left there unused and lonely. The reason it received so few signals was because it had no radials. Mounting a vertical high frequency (HF) antenna on the ground with no radials is about the worst case scenario when it come to mounting a vertical antenna. A good ground for RF radiation is not simply an eight foot copper rod driven into the ground because a vertical antenna wants to see a highly conductive field around it. Since a quarter wave vertical is essentially one leg of a half wave dipole oriented vertically, the radials act as a sort of "mirror". A better reflection so-to-speak is created with a better radial system. We can trick the antenna into thinking the soil surrounding it is in fact highly conductive with a field of radial wires extending out a quarter of a wavelength on the lowest frequency of operation
What Makes a Good RF Ground?
Copper plating the area around the antenna would be about as perfect a ground one could obtain. Yet for good reason I have yet to meet someone who has actually done this. It's impractical. Instead we can all thank the "skin effect" at radio frequencies which make building a solid conductor much easier for RF. High frequency radio waves see a metal screen as a solid conductor, this is why some TV satellite dishes are made of a mesh surface as opposed to a solid surface. Therefore, installing a screen or assortment of radial wires around the antenna will make is think the surrounding soil is very conductive.
There are marginal benefits associated with a ground wire radial system. Broadcasting stations will install about 120 radials for an antenna which can add up to miles of wire underground. One hundred and twenty radials are a lot of radials to bury, if your up to the task then best of wishes to you but most of us will be much happier with less. During my research I decided that 32 radials would be best due to the marginal benefit. Every radial after the 32nd brings a minescule gain in performance. However, I decided that 16 radials was a great starting place; sixteen radials offers a huge increase in antenna performance with a manageable amount of work.
Installing a radial system for a vertical antenna should be a weekend project for most people. Those of us with obstacles may find that it takes considerably longer. On one side I had a rock wall bordering my property line and on another a large compost pile. The compost hadn't been turned in years so it was compacted down very tight, covering about 100 square feet of yard; I didn't want to get rid of it so I ended up moving the whole pile over as I installed the radials and replacing it when completed. I had hundreds of feet of solid copper three conductor 14 gauge house wire left over from a garage we had built several years ago. Insulated and non insulated wire removed from the cable was used in my radial system without detrimental effects even though Newtronics Incorporated who makes the Hustler 5BTV suggests using insulated wire. It doesn't matter if the antenna mast makes a good ground connection with the soil since a radial system is not trying to do a grounding rod's job, it is the ground. The ground plate for the Hustler 5BTV was not used either; I simply used #10 solder lugs to attach each radial wire to the U bolt on the base as the pictures show.
Installing the Radials
The radials should be cut to the lowest operating band. In my case I don't plan on using this antenna on 80 meters (3.5-4.0 MHz) so I cut them for 40 meters (7.0-7.3 MHz). Each radial is 33 feet long and I installed 16 of them which makes the total amount of wire buried 528 feet. Like I said 16 radials looked like a great number to start with, just imagine 32 radials! It is ideal to pair each radial 180 degrees apart but I wouldn't worry about attaching one more here and there as time goes by. I didn't have the luxury of being able to install each one pair at a time given the location of the antenna so I placed each wire roughly 22 degrees apart. The goal wasn't to be exact and it doesn't make a huge difference if they are off a little; just eyeball it. My antenna is about ten feet from my property line causing me to deal with the innability to place several of the radials at full length.
You have the choice to bury the radials or lay them on the ground, whichever suits the circumstances presented to you. Local hardware and landscaping stores sell lawn staples that will hold the wire down enough so a lawn mower shouldn't hit them. I have been told that after a while the wires become totally covered with grass making them almost impossible to notice. Alternatively, I chose to bury the ground radials several inches below the surface. My idea was that if I did this the chance of tripping or sucking them up with a lawn mower was slim to none and they wouldn't be visible at all. In addition, burying the antenna radials was also a more "professional" approach to the project since they would be permanently in the ground. I saw the amount of time this project was going to take and I knew I wanted to do it the best I could, no skimping allowed.
As the photographs show, the approach was not to dig trenches because that was too time consuming and a waste of effort better spent elsewhere. The wire is relatively thin being 14 gauge solid copper so I chose to use a shovel, preferably a square edged shovel, to cut slits into the ground long enough to fit the length of the wire. Every now and then I would switch methods between cutting the slit the full length of the wire and cutting a few feet at a time; following that with installing a portion of the wire, repeating until done. Eventually a rhythm will form, streamlining the process of installation. To push the wire in, I found a piece of scrap steel lying around my garage.This method was perfect for lining the wire up on the ground above the slit and pushing the wire in several inches without disturbing the soil to much. After the wire is securely in place I just packed the lawn back down with the shovel. When all was done it was hard to tell that I even dug any soil up. After several weeks there were no signs whatsoever that the ground was disturbed, bonus points!
The best part of taking the time to install radials is when the project is done and you get to test it. Not only was the lowest obtainable SWR (standing wave ratio) significantly lower, the antenna was more stable when tuning the its elements. Signals being received by the antenna were significantly stronger too. Per instructions in the Hustler 5BTV manual I also installed two 1:1 coaxial choke baluns to isolate both the transmitter and antenna from the feedline. Due to the coax being located in the radiation field of the antenna it will have an induced current on the ouside of the shield causing SWR and radiation from the transmission line. Both were wound with ten turns of coax on a six and one half inch diameter paint can, followed with duct tape and wire ties to secure them.
I was up late one night and for some reason tuned around the 20 meter band (14.000 to 14.300 MHz in the U.S.) around 1:30 AM EST. Normally 20 meters is a "day" band that dies at night. Well, with the vertical antenna I was hearing Australian stations which I rarely hear in Massachusetts. A recording of the activity is on this audio file but I had no success at contacting Austrailia with 100 watts of power. Granted, me hearing the Austrailian stations had a lot to do with propagation conditions it was impressive at that time of day to say the least.In conclusion, this project was well worth the time invested. I started with a twenty-one foot tall aluminum pole stuck in the ground and turned it into an antenna! As time goes on I hope to add radials to further improve it's performance. Take my experience and apply it to your own antenna situation. If you don't have a vertical to do this with or haven't already done it I hope you can learn something from my experience; All in all, installing the radials on the ground mounted Hustler 5BTV antenna was rewarding not only with a great performing antenna but the experience gained by installing such an antenna system
|Last Updated on Sunday, 06 September 2009 14:43|