Saturday, December 21, 2013

Antennas, Towers and "Stuff"

Well, here we are four days before Christmas and the weather has been "WONDERFUL"! Today's temperature was in the high 60s with virtually no wind. Ditto for yesterday. Perfect for antenna/tower work. Unfortunately, Bill, KJ4EX, my tower guy, has one of those erratic work schedules and couldn't get time off to come over and finish the tower installation.

HOWEVER, Dave, N4KD, is in the midst of erecting a small tower and beam at his place. Since he has been one of my "go to guys" for my installation, I assured him that if he needed assistance or coolie labor, I was available. He completed his hole earlier last week using a rented auger from one of the big box stores. Bill, KJ4EX, had access to a similar auger which he used to dig my tower hole next to the house earlier in the year.

Using an auger greatly speeds and simplifies the hard work of digging out the hole that holds the concrete and the base of the tower. Once several holes are dug using the auger it is simple work to remove the excess dirt and square up the overall dimensions of the hole to make a square (or rectangle) approximately one cubic yard which will hold about 800 pounds (ten 80 pound bags) of concrete mix and the tower base. This type of installation is good enough to support a tower installation up to 80 or 90 feet, provided guy wires (or Dacron rope) is use to prevent lateral movement of the tower. In my tower installation I will be using one set of three guy ropes set at just under the 60 foot level on the tower. These will be attached to guy anchors (augers, actually) set out into the yard in three locations approximately 120 degrees apart. In addition, I am attaching the tower to the side of my house, just under the roof gable, at the 25 foot level for additional support.

While Dave didn't seem to need me for anything like "work" he did mention that since I had a cement mixer (actually a birthday present to Patricia, KB3MCT, a few years back) he would be tapping me and my mixer when he was ready to plant the base of the tower into the hole and add the concrete. Of course, I said "yes" so right now I am awaiting his phone call.

On to other antenna projects here at the Bent Dipole Ranch.....I procured two Newtronics (Hustler) 4BTV verticals, one just prior to Dayton from a local ham who had it stored in his shop, and the other one I procured new, from DX Engineering, at the Dayton Hamvention. The used antenna was in pretty sad shape, and I will be restoring it over the winter for use as a portable emergency HF vertical to support EmComm. The new one will be affixed to the roof tower that use to hold the Cushcraft A3S HF Yagi and the Skycraft 6 meter beam, which are being moved to the new tower after completion of the tower installation.

The 4BTV is a rather interesting antenna. Originally designed as a quarter wave length trapped vertical to cover 40/20/15/10 meters, this antenna has enjoyed a dedicated following over the years. I used one when I was active in the Azores (CT2BH) in the early 1970s with outstanding results. Over the years the engineers have modified the design to include adding the 80 and 30 meter bands, using additional traps and resonator coils. 80 meter operation is quite restricted due to the limited bandwidth of this vertical's design. Although I have never had one modified for 30 meters; I may add that trap (available from DX Engineering) before erecting it on the roof tower.

One thing to always remember regarding vertical antennas: they need a good radial or counterpoise system to properly radiate an RF signal. Since the vertical antenna element itself is only a quarter wavelength radiator, the radial system provides the other half of the antenna. Since this installation will be above ground on my roof, I will need to provide wire radials cut to approximately a quarter wavelength of the operating frequency on each band. According to the manufacturer I will need at least two, possibly four, of these wire radials per band! This makes for a lot of wire on the roof! In my Azores installation (roof mounted) I used four radials per band and the antenna worked amazingly well. Therefore, I will duplicate that counterpoise system at my current location, in hopes of having an antenna that is a real "DX getter".

The radials don't need to be anything fancy. In the past, on other vertical installations, I have used wire sizes ranging from #26 to #12 with good results. There is also a method of taking five conductor rotator wire and cutting the radials for up to four bands. This greatly simplifies the number of wires you have to deal with and terminate on the roof. I have used this method once before and it worked well. Depending upon my situation here at the Bent Dipole Ranch, I may use that method again just to save space.

One thing about multi-band trapped verticals:  adjustment of the antenna and the traps is paramount to obtaining good results on the air. Using my trusty MFJ Model 269 antenna analyzer and taking my time, I will insure that the element measurements are accurate and the radials are properly cut and configured.

Many hams who erect a quarter wavelength trapped vertical and fail to put the necessary work into the counterpoise system see a very low SWR across the bands. This leads them to the erroneous conclusion  that their antenna is working just fine. On the contrary, this means that the radiation resistance is so low that the antenna won't radiate properly on the air. You actually want to see a pronounced "dip" in the SWR curve on each band. Remember: the higher bands won't exhibit as much of a drastic "dip" as these bands tend to be much wider in working bandwidth than the lower bands. On the 4BTV the 40 meter band should only be about 75 to 100 kHz wide, from the chosen frequency of operation to where the SWR rises to approximately 2:1. On 80 meters (provided you add the capacity hat and 80 meter resonator) your bandwidth will only be about 25-30 kHz! This means that you will have to re-tune your antenna tuner if your operating excursions exceed the chosen frequency of operation.

One of the major features regarding quarter wavelength verticals is that, when properly configured and adjusted, their angle of radiation (the take off angle of the radiated RF wave) is very low, allowing the RF signal to project further out into space before it encounters a reflective layer of the ionosphere, where it will be refracted (bent) back toward the earth. This is refereed to as "skip zone" and the longer that skip zone, the further out you can successfully work other stations. So basically the lower the angle of radiation (take off angle) the further you can reach with your signals. Therefore, the vertical antenna is often refereed to as a good antenna for DXing. Again, this is provided the antenna is carefully installed and a proper RF counterpoise (radial) system is installed and tuned.

OK, enough for now. I need to get into the shack and do some much needed house cleaning and straightening up. Then, if time permits, I may get on the air and see what DX is available.

Until next time, Vy 73 and good DX.

Rich K7SZ

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