Sunday, May 31, 2009

Summer Means Its Time to Fool with Antennas

I saw one of these on virtually unused for a nice price, so I bought it. I'm not a big antenna tuner kind of guy. I pretty much build my antennas to provide a proper load to the coax. I do use some antenna that have wild impedance, but I I usually put some kind of matching network at the base of the antenna to bring it into line. My 160 ant for example has a base impedance of about ten ohms so I have a little network that brings that up to something around 45 ohms. I have a 40M vertical that has an impdeance of about 2000 ohms. I have a tuner at the base of the vertical that brings the antenna down to 53 ohms. All of my antennas have their own dedicated matching device that is built precisely to give me the most efficiency in my system. I have one antenna thats a 135ft flat top fed with 300 ohm transmitting line that comes into my Johnson Matchbox and I use the flat top all the way to 6M. On 6 I built a separate link coupled tuner so I just pull the Johnson out of line and plug in the 6M job and have at it. Again each band is optimized.

I recently put up a RX only vertical to play with the diversity branch of the test code. N4HY has added an interesting feature to PowerSDR that allows very impressive diversity peaking or notching that is steerable from the software. I have 2 full size 80M/40M/30M verticals already on the property, but I wanted to try a triangular array switching between any pair of 3 verticals to see if I could improve steering. The answer to that question by the way is yes. There are noise sources that you can null better on one pair than on the other 2.

My thrid vertical consists of a 45 ft piece of #14 that is suspended by a tree. When I first moved to this QTH in the mid 90's I had a tree man climb a few of my pines and put big old eye bolts into the tree, and I ran some steel cable through the eyebolts to the ground. This way I can use the trees to hoist up my antenna pipe dreams at will. For this antenna I merely tied an insulator to one side of the halyard tied the wire to the other and hoisted up the vertical. At the base of the tree I pounded a stake into the ground and to the stake I mounted a bracket that has a SO239 and some bolts which to hook radials to. Since this is a RX antenna I only put out 3 radials. So I have a 45ft piece of wire that is virtually invisible suspended from a tree in the front yard.

Of course I could not resist pumping some RF into this vertical. It didn't match very well, so I put the new little auto tuner in the line. This little tuner will tune automatically if the SWR is greater than some present number like 2:1. To see if this thing would even work I brought up and proceeded to send some CW on 80 40 and 30. On 80M the antenna is down several dBm compared to my full size 1/4 wave vertical with a very well characterized ground. The base impedance of that antenna is about 33 ohms which is near perfect. With the Flex 5K since I can rapidly change antennas I was able to switch back and forth using WEBSDR as my field strength meter 4500 miles away. Both signals were barely out of the noise as it was clear there is a lot of summertime static over in PA3 land. Band noise was running -100dBm over there an I was hitting as much as -93dBm. (I love being able to talk about signal strenght in terms of dBm instead of dorky S-Units) The full size super efficient 80M was louder by maybe 3-5 dB on the average than the front yard kluge but I could copy both antenna at 100W, and there were some QSB peaks where the antennas were were equal on the WEBSDR dBm meter. On 40 there was at best 1-3 dB difference between my 1/2 wave end fed super efficient flame thrower and this 45 ft piece of wire (about 3/8 wave on 40). On 40 this antenna should really shine, so what I am seeing is the poor efficiency of my front yard gound plane. On 30 I was able to tune up fine. After I was past my terminator and it got good and dark, I was able to hear both my 5/8 wave 30M vertical AND the 45ft antenna (which would behave like a 1/2 wave end fed on this band). The 5/8 wave vertical with the really good ground was louder. I was feeding the 45 ft piece of wire with about 75 ft of coax so it doesn't surprise me. My hope is to move the tuner out to the antenna thereby removing the "coax behaving as a attenuator instead of a feedline" from the equation.

So what's the upshot of all this? Well I've read a lot of hubub about those 43ft verticals. A lot of people swear by them and a lot of people swear at them. Mostly I think people are talking out of their sleves without ever actually trying one. This 45 ft invisible piece of wire (which is virtually the same as a 43ft aluminum pipe) is a hell of an antenna. You basically head down to the home depot and buy a 500ft roll of #14, spread all that wire out, and hire some joker to put an eyebolt in the tree. That's maybe 50 bux. Since this tuner has batteries, and the batteries that I have in there are slated to last about 4 years, and this thing is automatic, I am going to find a water tight box and put that sucker out at the tree and see what it does. I bet it will do quite nicely. I may add a few more radials as well. This antenna could well be the answer to those among us that suffer covenants and desire to put up a very good antenna that also can NOT easily be spotted. Basically someone would have to actually be in your yard up close to the tree to get a load of this antenna. At the base of the antenna I wound a loose coil about 4" in dia maybe 4 turns for strain relief so if the wind blows the tree around it just stretches out the coil a little. After all tomorrow is the first day of hurricane season.

I tried to load the antenna on 160 but even with the monumental ground loss I could not get a match. HOWEVER, there is nothing that precludes you from running a second wire up the tree in an inverted-L kind of scenario and switch between the 2. I use a coax switch out at the antenna to switch between 4 antenna using the coax as the means to control the antennas. Here is a home brew version by Phil Salas AD5X that allows for switching 2 antennas and it would be easy enough to increase this simple switch to 3 outputs.


To add a third position just add another relay and use diode steering on the relay coils. For position 1 use a + 12V, for position 2 reverse the polarity and use a -12V, and for position 3 use Zero volts. You would have to pick off the control voltages before the feed line went into the remote tuner, but you would want the antenna switch AFTER the tuner so you could use the tuner on all antennas. This sticks with my philosophy of tuning the antenna AT THE ANTENNA where ever possible. Also by using the outputs available via DDUTIL you could make the band switch to be controlled by PowerSDR as well, making band change virtually point and shoot!!! I wont go into what is involved there but it certainly is do-able.

This 45ft wire works great for 80, 40, and 30. It gets too long to be a good DX antenna above 30, and its too short to match on 160. I usually don't operate above 30M, but if you added a second wire to one of the positions of that antenna switch about 15 ft long you should be able to tune that second wire from 20 through 10 with good result, and if you could sneak a third wire up, some kind of inv-L about 130 ft in total length, or a T antenna you could get on 160 as well. You may need to buy a second roll of #14 for a few more radials if your going all out 160-10. (Actually I'm a fan of something around 3000ft in total radial lenght)

If your anygood with a fishing pole this antenna would be very easy to put up as a portable antenna as well. Just pop some line over the canopy of some unsuspecting tree and hoist and string out a few radials. Hook up the tuner and have a blast.

I've been measuring the distance between my trees. One is 60 ft away, another is 30ft away, one is about 100ft away. In fact I have 4 trees 60 ft apart!! Just the ticket for some invisible vertical arrays!! If I can only figure out how to get the coax UNDER the driveway. And I can even make DDUTIL control that from the computer!!

Covenants? What covenants?