The AM Forum

Looking at the Bobtail on 20 meters. See EZNEC run.
The antenna input Z at about 20 feet looks like 4500
ohms and slightly reactive. The handbook illustrates a
simple tapped inductor match and tuned resonant network.
I ran a quick simulation and indeed about 8 uH tapped
at 2.7 uH and a 24 pF tuned C works out nicely.
However, and I have not seen this, a 1/4 wave
450 ohm ladder line will do a fine job of direct match
to 50 ohms. Has anyone done this? Am I missing something?
And yes I know, it 's only a model.  :) Thanks! Alan

You can use a qtr wave of any impedance line.   Somewhere within that qtr wave is a 50 ohm tap.

That's the theory behind J pole,  zepp etc.

--Shane
KD6VXI

A four-element Bruce Array has just as much gain but requires less space and needs no matching. It also has a larger bandwidth. Just something to consider.

Thanks Steve. I'll take a look at the Bruce Array.

Steve, the takeoff angle is about the same as the Bobtail, ~ 15 degrees. I looked at the 2 element as mechanically it is certainly simple. The gain is a bit less than the 3 element Bobtail and the pattern appears a bit more sensitive with height. Yes, the Zin fed from the center of one of the long legs is nice. However, I understand feeding the Bobtail from its corner or its center and eliminating one of the vertical elements, gets you close to 50 ohms. It is smaller than the Bobtail. Thanks for the heads up.

Check out the "Field Day Special" antenna too.

Hi Alan,

When ezNec modeling first became available to hams in the late 80's / early 90's, I tried every antenna known to man, especially on 75, 40 and 20M.  I had a 150' Rohn 45 tower to work with and used rope and Phillystran to support all kinds of wire arrays.  I went through 18,000' of #5 copper clad aluminum wire testing the models I came up with on the computer..

Bottom line, on 20M: The very best array was a lazy H with reflectors 1/4 wave behind it. (8 elements) The stacking height was 5/8 wavelength spacing.  I went on to extend the Lazy H legs until at one point I had 36 elements on 20M, beaming Europe.  I used a simple reference dipole for quick A/B tests on the air and it was like not hearing a station to S9 at times. Unbelievable results.

The good thing is the modest height required -- Lazy H with the top at 70' and bottom at 35' worked well.  I finally settled on the top at 90'.

I fed the Lazy H with open wire in the center between the top and lower elements for balance - and matched it to OWL to a 50 hardline/balun once  I found the 50 ohm point along the OWL.

You will generally find that horizontal beams do better than verticals on 20M. That's why all the corntesters use Yagis and not vertical arrays.  (160M is the exception)

T

Hi Tom and all... I completely agree. However, bottom line, I was looking towards an easy but reasonable return on the effort antenna superior to a ground mounted vertical with 32 radials. It works, but no great excitement there. The Bobtail from a mechanical point is an easy extension from the vertical with the vertical serving as the feed. The Lazy H I considered, but the mechanics with all the tress in my yard was going to be an exercise in frustration.

Try moving the feed point to the top corner. Using these numbers shud get you close and the 2:1 SWR bandwidth is 13.8 - 14.6 MHz.

Frequency = 14.2 MHz

Source 1      Voltage = 48.09 V at -3.95 deg.
              Current = 1 A at 0.0 deg.
              Impedance = 47.98 - J 3.309 ohms
              Power = 47.98 watts
              SWR (50 ohm system) =  1.082

Thanks! Yes, the top corner is a good solution. I was concerned about arrangement of the coax. I believe a current balun is still warranted there as well. Ideal, the coax does not enter into affecting the pattern. Can the coax drop straight down and parallel to the vertical element? This is something I was going to look at,  handling the coax properly in the installation. Again, the only benefit of bottom feed and I no it is a small issue, is having to drag a cable up in the air.  :-\

On the ones I've built in the past I ran the coax away on the support rope for that corner over to the tree the rope was in then down the tree. It was positioned 32 feet from the tree.... @ a half wave.

That's a half-square and I would consider it a better option than the Bobtail also. N6LF has some good info on this antenna too.

http://rudys.typepad.com/ant/files/antenna_halfsquare_array.pdf


A fair comparison is the Half Square and the 3-element Bruce, since they are the same size.

All that said, I'm curious about your goal/requirement for your 20 meter antenna. The value of a vertically polarized antenna decreases as frequency increases. You can put a horizontal antenna far enough above the ground (relative to wavelength) at the higher frequencies to obtain a low takeoff angle. And, you will not suffer the ground losses you would with a vertical.

Look at the sim plots below. They compare a Half-Square with the top at 30 feet above average ground to a dipole at 30 feet above average ground. The dotted lines are the Half Square, the red line is the dipole. Yes, the dipole has more high angle radiation and the Half Square has a lower takeoff angle. But, at the takeoff angle of the Half Square (15 degrees) the dipole is just as good.

I just didn't draw in the 3rd element Steve but you are right. Since the verticals are 1/2 wave spaced the feed point results will be similar.

Half square would give a nice 2 lobe while the 3rd element would produce some kind of 4 lober. 'Course you knew that! :)

All that said, I'm curious about your goal/requirement for your 20 meter antenna


Hi Steve, thanks and very good on the information. The goal was install a simple antenna arrangement with little effort and somewhat hidden. As I have HOA to deal with I cannot put up a tower and yagi or half square or Lazy H at 80 feet, without drawing attention. I have a homebrew 3 element yagi, a shorty 20 but even it is too obvious. SO SIMPLE WAS THE GUIDE LINE, IDEAL NO RAISED FEEDLINE, antenna all bottom fed. Currently I am running inverted L's on 30 and 40 which are 3/4 wavelength long and work ok, bottom fed, using radials and the vertical is on 20.

So in summary, simple to erect, ideal feed line at the base of the antenna... So the Bobtail seemed to be the proper choice with some gain in the horizontal, decent take off angle, bottom fed with a LC tuned network or a TLINE match. Mechanically, no more problematic  than a dipole to erect. Thanks!   

The bobtail fed at the center element will have a 1:2:1 current distribution among the vertical elements. This produces a little more gain and narrower main lobe. Not sure if it's the same with corner feed.

Alan, thanks for passing along your requirements. The bobtail does seem to fit your requirements well. Let us know how it works out.