Topband: how high are very high voltages on antennas?

[Grant Saviers]

Thanks for the insight.  I will try the EZNEC suggestion.

"stub" was a bad choice of words, I've got two approaches in 
contemplation/analysis mode, both wire loading schemes:

1. Four relay switched vertical wires at the center bottom voltage 
node.  The antenna has about a 25 KHz 2:1 bandwidth so a "binary" ladder 
of 4 wires can cover the whole band if the natural loop is resonant at 
about 2Mhz.  Of course, the wires are not 8:4:2:1 in ratio, so I've been 
trying a lot of length combinations to get full, no gap coverage and 
have a compromise set that "works".  I've also got  a continuous tuning 
controller scoped out to sequence the relays.

2. a metal spool and contact system to feed out hi-flex Cu bare wire and 
dacron pull up line on a plastic spool on small (screwdriver) motor with 
rotation count output (both spools on a single delrin shaft).  Wire out 
one side, line in the other to a pulley hung from the apex that lowers 
with the antenna.  An MFJ screwdriver controller provides 10 preset 
turns counts, established at 20KHz steps by manually measuring swr.  I 
use the MFJ controller on my Tornado variable inductor tuned 80m 
rotatable dipole (rebuilt EF-180C) for full band coverage and it has 
been solid.

Option #2 is gaining on #1 in my mind.  I could buy a SteppIR 40m sized 
vertical to do the variable loading, but that is a bit pricey!

Anything like this by other topbanders?

My tree climber peaked out at 87' for the apex pulley support.  It seems 
to me that the vertically polarized delta loop is the best single 
support choice for 160m  < 30 degree gain with a little directivity,  
and a no extensive radial system.    Comments?

Happy Turkey Day!

Grant KZ1W


On 11/22/2012 5:27 AM, Tom W8JI wrote:
>> It is often mentioned that "very high RF voltages" are present at 
>> ends of dipoles, voltage nodes on loops, etc. but I haven't been able 
>> to find any guidance in the form of numbers.  How high is "very high"?
>
> An antenna is also a transmission line, and so the voltage depends on 
> losses along that line, the line surge impedance, and the current 
> along the line.
>
> This is why thin wires in antennas (antennaE are on insects, antennaS 
> are on radios) with low radiation resistance have very high voltages, 
> especially when loss is low. Since radiation resistance of a full wave 
> loop is fairly high, voltages will not be extreme. A delta is a little 
> worse than an expanded area loop with a little less gain, but not 
> much. All are about equal to a regular dipole.
>
> http://www.w8ji.com/delta_loop.htm
>
>
>> Assuming QRO power into a full size apex up delta loop, corner fed, 
>> vertically polarized, what will be the voltage at the apex?  I 
>> haven't found a way as yet to have EZNEC give me a value.
>
> Connect a second current source to a small wire running away 
> vertically from the corner for 1/8th wave, and set the source to zero 
> current. The voltage given in the Source Data button tells you the 
> voltage.
>
>> If I wanted to put a relay at a voltage point to switch a stub, how 
>> would I float the 12v DC coil from the RF present?
>
> Why would you switch in a stub? Switching in a "hanging" wire at the 
> apex or at the center of the bottom horizontal wire (at a voltage 
> maximum) will lower the resonant frequency. Or are you switching in a 
> long stub, to make it an entirely different mode of operation?
>
> 73 Tom
>