Monday, June 20, 2011

Secrets Of Getting a Vertical To Work

Secrets Of Getting a Vertical To Work - Or How I Overcame The Old Saw, "A Vertical Radiates Equally Poorly In All Directions"

Seems like everyone has an opinion about verticals. They either love 'em or hate 'em. Those who hate 'em say the best helpful hint is to throw them away. Those who love 'em say (and I count myself on this side) you just need to understand a few things.

Right off let me say that I am not the expert but I have studied a lot of documentation and I have some favorite resources. A lot of credit goes to my good friend Bill Rinker, W6OAV for getting me thinking about verticals during a trip to Dayton.

The light eventually came on for me and I put a small list of "revelations" together:

1. A big top hat (like the one in this picture) helps a lot. It raises the radiation resistance which makes a vertical easier to match.

2. Near field determines efficiency and far field determines angle of radiation.

It's pretty obvious you need a good ground system but do you need as much as I have in this picture?

3. Since the near field determines efficiency, losses here are very important. Ground loss is one of those losses, and are a function of the ground system. Soil conditions determine how many radials are needed to keep the near field losses at a mininum.

4. Soil conditions also affect the far field and the angle of radiation but we can't change that, other than to pick a location with good soil when we are looking for real estate.

5. Good traps don't waste significant power. Coax traps are not the best but they are not very bad.

6. Shortened verticals have the same radiation angle as a full size quarter-wave vertical, but shortened verticals are less efficient. Verticals longer than a quarter wave have gain.

7. Shortened verticals have narrower bandwidth and lower resistance.

8. The radius of your ground system need only be as big as your vertical is tall. Shortened verticals don't need a full-size quarter wave ground screen.

9. Low power Hustler coils radiate better than high power Hustler coils due to capacity coupling from their end caps, but it's barely measurable.

Ok, I've thrown out some possibly surprising statements here. Do I detect any skeptical thinking after reading this list? I'd like to reveal one of the authors who I believe is very expert and the one who has contributed many of the above statements. His name is Jerry Sevick,W2FMI and he is widely published. Dr. Sevick's works are a must read for any vertical antenna enthusiast.

If you don't recognize the name Sevick you might remember this antenna---the famous 6 foot 40 meter vertical. This antenna was first published in QST in the mid-1970's and is documented in several places, including The Short Vertical Antenna and Ground Radial, Sevick, CQ Communications, Inc.,2003. I don't mean to diminish anyone else's publications. What makes Dr. Sevick's publications special and why I respect Dr. Sevick's work is this. He doesn't take anyone's word for it. He builds it himself and he measures the results himself. And then he documents the results and publishes them. That's a tall order. Dr. Sevick's career was at Bell Labs where that kind of "prove it to me" mentality is typical.

Next, I will discuss how I constructed my vertical and how I made my measurements.

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.

Keeping It In Perspective
Twenty three per cent of the power was being wasted heating the ground before I added the new radials. It was like working the DX with 77 watts which if you stop and think aobut it, really is enough to work a lot of dx. Converting that loss to S units gives it some more perspective. One S unit is supposedly 6 db or 4 times the power because power doubles each 3 db. If 3db equals double the power, a change of 23 per cent must be less than 1 db. That would mean 23 watts makes about 1/6 of an S unit of difference. Do you think anyone can detect a difference that small? Keep things in perspective before you spend a lot of effort putting down 46 more radials.

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.

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.

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