[TowerTalk] Broadband Sloper

[K4SAV]

W5CPT wrote: "The author of the article claims an almost omnidirectional 
pattern at about 30 to 60 degrees.  The mininec charts are in the book."...

Don't pay too much attention to those Mininec plots in the Antenna 
Book.  Those are for a perfect ground.  They show about 5.5 dBi gain at 
zero elevation angle. Worthless is what they are.

This is an interesting antenna.  I first looked to see if there was any 
info on the internet on this antenna but found none, and the Antenna 
Book was no help.  It just said here is an antenna. I thought for sure 
that the broad-banding was going to be achieved by large ground losses.  
There IS a lot of ground loss but the antenna achieves a double 
resonance point which helps make it very broad-banded.  Placing one 
resonant point at 3.5 MHz and another at 4 MHz makes for low SWR at 
those frequencies.  Additional ground loss helps to flatten the curve in 
between those frequencies.

For those who care, here is a description of what I found.

It's difficult to describe exactly how the multiple resonance points are 
achieved.  You can see that it is a three pole antenna.  One pole going 
up the tower, one pole going down the tower, and one pole going out the 
horizontal vee wires.  Those wires all interact, but in general the high 
resonant point is mostly affected by the top sloping wire (plus Yagi top 
loading and height of the tower).  The low resonant point is mostly 
affected by the vee wires and the attachment point on the tower.  Both 
of the resonance points depend on the connection to ground.  If the 
lower portion of the tower is decoupled with a trap, there are NO 
resonant points in the 80 meter band.  (Which means you can't improve 
efficiency by trapping out the current going into the dirt.)

I looked at the 50 foot tower version of this antenna shown in the ARRL 
Antenna Book (17th edition).  I had to juggle the lengths a little to 
get the resonant points at the right frequencies.  I didn't get the 
exact SWR curve they measured, but it did have an SWR of less than 1.5 
from 3.5 to 4 MHz and it had the little peak at 3.8 like they measured.  
Close enough.  I used a ground loss of 30 ohms. That's a poor ground for 
sure, but you might get that on a tower with no radials. The gain at 3.5 
MHz was -5.6 dBi at 32 degrees elevation (-8.4 dBi at 10 degrees 
elevation).  The gain at 4 MHz was -1.4 dBi at 45 degrees elevation 
(-4.7 dBi at 10 degrees elevation).  It's obvious that there is 
significantly different ground currents near the two resonant points. 
For 100 watts of applied power the near field ground loss at 3.5 MHz was 
66 watts.  At 4 MHz it was 28 watts.  Those losses don't account for the 
4.3 dB difference in gain.  The pattern changes also.  It is more 
omni-directional at 3.5 MHz, and at 4 MHz the extra F/B helps out the 
forward gain.

On 3.5 MHz the current amplitude on the tower is small for most of the 
length of the tower.  There is a current maximum near the Yagi, and at 
the top of the loading wire.  There is more current going into the dirt 
than at any other place on the antenna.  On 4 MHz there is significantly 
more current in the tower and the sloping wire from the top.  However 
once again there is more current going into the dirt than at any other 
place on the antenna, although less than at 3.5 MHz.

You could add a good set of ground radials to the tower and reduce some 
of the ground loss and improve the gain.  The SWR won't be as flat.  The 
Antenna Book said that when this was tried the SWR was still under 2.0 
across the whole band.  I can believe that looking at my data.  With 8 
ohms of ground loss, the gain at 3.5 MHz was -3 dBi at 29 degrees 
elevation (-5.5 dBi at 10 degrees elevation), and at 4 MHz it was -0.3 
dBi at 39 degrees elevation (-3.5 dBi at 10 degrees elevation).  Near 
field ground loss was 39 watts at 3.5 MHz and 10 watts at 4 MHz.

Jerry, K4SAV

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