[TowerTalk] High power 2:1 transformer was Re: 16om vertical and the number of radials

[Grant Saviers]

It is true that gain changes a few tenths of a db over the range of 
radial heights.  Antenna efficiency matters more than gain changes for 
verticals near earth.

The height matters a great deal below 10ft high.  It is less sensitive 
the more radials.  My antenna is a T, 77ft of vertical wire and two 36ft 
top hats with 8x 125ft radials at 10ft.

So here is a comparison for my antenna R (=Rr+Rg) over average ground 
for 2 x 125ft vs 8 x 125ft.  Modeled EZNEC Pro/4.2 with radials at 
different heights over average ground:

ht-ft	2 rad	8 rad
5	33	25.2
10	33	24.3
15	30	23.5
20	29	22.8
25	28	22
50		19
75		17
100		14

A 2:1 TLT matching transformer shows a 1.1:1 SWR at resonance for my 
antenna with radials at 10ft.

The difference in efficiency between the 2 and 8 radials at 10 ft is 
1.38db, using K8YC 10log(ohm ratio) formula. For my 8 radial antenna the 
Rg loss difference is 0.28db between 10ft and 20ft high.

The difference in efficiency between my 8 radial as built and free space 
(100ft proxy) is 2.34 db.    The take off angle also changes from 23* to 
15*.  A plan B vs near/over salt water is put the antenna on top of a 10 
story building. ;)

So my conclusion during design (tree supported T in a forest) was the 
effort/performance was just fine at 10ft. Plus being in a forest causes 
more near field radiation loss - maybe 4 or more db.  As one poster 
said, "all the trees are cut down around BCB antennas".

The advice to have elevated radials at least as high as 10% of the 
wavelength seems fairly practical for higher bands.  Likewise, more 
radials are better until as Rudy showed, there isn't much benefit >8.

With a coil loaded short radials 2 radial DXpedition antenna we tested 
the difference between 4 ft and 12ft was on the order of 10 ohms. Again, 
height matters most with few radials, more so with shortened ones.

Grant KZ1W



On 12/3/2022 15:00, Artek Manuals wrote:
> If you read N6LF's work closely (see fig16 in 3/2012 QEX) you will see 
> that radial height above 5' adds a few 10ths of a db improvement at 
> most. (8ft =.015 wavelength above ground) Given noise and QSB on 160 
> hardly worth the effort ..IMO.  Ditto on the number of radials more than 
> 4 at a reasonable (8') height buys very little as well. What is actually 
> more important is soil conductivity  Rich farm loam vs sandy Florida 
> (Fig 15)  is worth 5DB !!! I cant imagine trying to keep sixteen 134' 
> radials 20 feet in the air !
> 
> Another good read on elevated radials ( non resonant radials) that 
> doesn't get much play is by K5IU, "Optimal Elevated Radial Vertical 
> Antennas" , Communication Qrtly, spring 1997. If Google is not your 
> friend contact me off list and I will send you a copy.
> 
> Dave NR1DX
> 
> 
> On 12/3/2022 2:17 PM, Jim Brown wrote:
>> On 12/3/2022 10:20 AM, Chuck Dietz wrote:
>>> Thanks for the info. I think I am going with 60 radials on the ground. I
>>> want to use the 160 vertical tower to support an 80 meter array 
>>> around it.
>>> Elevated radials might make adjustments way more complicated. 
>>
>> It's worth studying N6LF's work on elevated radials. He stresses that 
>> keeping radial currents equal reduces loss, that making them slightly 
>> shorter than resonant helps that, and so does having MORE elevated 
>> radials -- for example, 8 is better than four. From N6BT, I learned 
>> that elevated radials for 160M should be at least 16-20 ft high; I 
>> learned that when I asked him why my 4 ft high radials weren't working 
>> well.
>>
>>> I have a
>>> tractor and a good welder guy to weld a bracket and tube to a single 
>>> plow
>>> shear to make a radial plow.
>>
>> Remember that the only virtue of burying radials is to protect them 
>> from damage, or from being a trip hazard. We are NOT trying to couple 
>> the antenna to the earth. The function of radials is to SHIELD the 
>> field produced by the antenna from the lossy earth, and to provide a 
>> low resistance path for the antenna's return current IN PLACE OF THE 
>> LOSSY EARTH.
>>
>> Radials DO couple to the earth, and the ground loss shows up as series 
>> resistance. But radial current divides by the number of radials, power 
>> loss is I-squared R, so gets smaller in each radial twice as fast as 
>> the number of radials is increased, so more radials reduces loss.
>>
>> 73, Jim K9YC
>>
>>
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