I built two radial systems for this same 23' vertical and moved the antenna back and forth. One radial system
is made up of 64 wires extending from
an aluminum disk as shown in part 1 of this article. The other consists of four radials made of welded-wire fencing laid flat in an X pattern. Each is 25' long.
With a perfectly reflecting ground the resistance should be 36 ohms or
one half of a what a dipole is. Using an MFJ-259 analyzer both radial systems yielded the same measurement,
36 ohms, at resonance, indicating a perfectly reflecting ground, at least within the limits of the analyzer's accuracy.
Based on thexe measurements I conclude that it doesn't matter
whether you use fencing or 64 wires. However, fencing is much easier to lay down.
At one time I had only 18
wire radials and the resistance measured 47 ohms, indicating a considerable ground loss. It was easy
to match and I did work a lot of DX, even with the losses. How much was I losing and how much was
getting out?
I calculated power lost as I-squared-R, where I is current and
R is resistance. If we operate only at resonance we have only resistance to deal with and
don't have to be concerned with reactance. Power is lost as heat, and the loss is heating up the soil
slightly at the base of the antenna.
With 100 watts, solving for I the nominal current at 36 ohms would be 1.67 Amps (Square root of 100/36).
At 47 ohms the same
power is delivered from the transmitter assuming a matching network or tuner is virtually lossless and is presenting a 50 ohm load
to the transmitter both times. Again solving for I, the current is now 1.46 Amps (Square root of 100/47).
Before, there was only one
resistor and that is the radiation resistance of 36 ohms. Now, there are two resistors, the 36 ohms
of radiation resistance which doesn't change, and now the 11 ohms of ground loss. The difference is the portion of
power wasted, probably as heat generated in the ground loss resistance. That loss can be calculated as I-squared-R in the
11 ohm resistor, or
(1.46 * 1.46 * 11). The result is 23.4 Watts, or about 23 per cent of the 100 watts.
As the analyzer shows, it is possible to obtain a reading of 36 ohms. One doesn't always need the broadcast standard of 120 radials to achieve a perfect ground. A lot depends on soil conditions. The better soil conduction the fewer radials you need. If you don't your soil condition you can let the analyzer show when you have enough. Over poor soil one probably would need 100 and over salt water one needs very few, if any. At my QTH 64 radials appeared as a perfectly reflecting ground.
Top Hat
The vertical dipped at 10 MHz which is great for 30m but I was trying to build a 40 meter antenna.
Making the antenna taller was not an option so I needed some sort of loading. I had read a top hat adds loading without
adding loss like a coil.
I added six top hat radials by salvaging three
elements from an old 2 meter beam. The diameter is 38 inches. The resonant frequency of the original 23' vertical measured 10.0 mHz.
This addition lowered the resonant frequency, but only to a disappointng 9.0 mHz.
Next I tried adding a ring of aluminum wire using small hose clamps to hold the wire to the tip of the top hat elements. Now
the resonant frequency measured 7.8 mHz and this is where I left it. Resistance at 7.8 mHz
measures 36 ohms on the MFJ analyzer.
If I want the antenna resonant at 7.0 mHz my next move is going to
have to be the introduction of some other kind of loading such as a coil which I would put about two thirds of the way up. Or I could figure out a larger top hat.
Meanwhile, I am temporarily coupling the transmission line to the antenna with a SGC-230 Smartuner. The Smartuner forgives all shortcomings. It adds
inductance to tune out the capacitive reactance present due to a higher resonant frequency. It
also transforms the 36 ohms resistance to 50 ohms for the transmitter.
Snapshots of the fencing radial
system, the split bolt connector I used to connect to it, and the Smartuner.
Footnote: In 2006, I replaced this vertical with a commercial version from DX Engineering (Model DXE-40VA-1). The top hat is bigger and the whole antenna is physically stronger. It works great. One big advantage to me is, I can use higher power if I need to. On 40m full legal power can produce a lot more dx contacts. Now, in 2007, I am thinking about putting up a second one and phasing it. The more I read about phased arrays the more I realized how tricky the phasing is. I have not decided one way or the other at this point.
2011 Update: In the years since, I have revisited this original 40m vertical. Since the top hat made the antenna physically unstable, I removed it. I now have the same vertical extended up to 32 feet. It now resonates at 7.0MHz without loading. I ignore the mismatch of 36 ohms versus the perfect 50 ohms and the antenna performs well anyway. I am using dacron guys to stabilize it and the wind has not blown it down yet. Meanwhile the DX Engineering 40m vertical has been modified. It is now the lower portion of an 80m vertical. I extended a wire from the tip of the original vertical up to my tower. It needed enough wire to make the vertical resonate at 3.5MHz. The wire tilts at a 42 degree angle which apparently adds some capacitance to ground. Instead of resonating at 67 feet for 80m it needs lengthening. It needs 75 feet to resonate. Next, I modelled this antenna with EZNEC. The resulting pattern shows the tilted wire destroys the nice donut shape radiation pattern and makes it a cloudburner. I worked a lot of contacts with this antenna on 80m but I imagine not as many as I would have if the whole thing had been vertical. I have no way of making it totally vertical so we'll never know.` Finally I added a 30m half wave vertical sloping down from the tower. This antenna is a PAR Electronics PAR-30. The half wave works well on 30m. It is about 20 feet away from the tip of the 80m vertical and about 30 feet away from the tower. Unfortunately it is close enough to couple to the 80m antenna. The resonance has lowered to 3.4MHz. The 30m antenna is apparently acting as a top hat, despite being 20 feet away. I am living with the compromise for now.
The WAF was low on this arrangement (wife acceptance factor). I have now reverted to using the DX Engineering vertical without the top hat as a free standing multiband antenna. I have the SGC tuner at the base and no guy wires. This works well for 40m through 10m. For 80m I put up a 67 foot sloping wire. For the RF ground I am using one elevated radial. It hasn't been up long enough to test it very extensively but so far it looks good. The measurements look promising.
Again I modelled this antenna. Modelling shows a different pattern on each band and the only band with the nice donut pattern is where the antenna is resonant. The other bands have various distortions, mostly bad for dx radiation. All bands would be good for state-side contacts however.
The appearance from the street is better. The 80m vertical hardly shows and with no guy wires or top loading the 23 foot vertical has a cleaner appearance. So far the WAF is in the acceptable range. Whew.
I ran across an antenna where the ham had wrapped the vertical with camo duct tape. It looked good in the pictures, just like a slender pine tree. I might try that here. One difference is his was nested in some pine trees and mine is out in the open. Hmmm.
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