[Lowfer] Active Whips and Rubber Sheets

[Tom Rauch]

 Les,

I'm not sure my mailer will allow me to post under my subscribed 
name, but I'll try again..
 
> I don't know all the details about the theory either, but in a
> nutshell it basically states that for best performance an active whip
> needs to be the highest object for at least a 1/2 wavelength at the
> lowest frequency of operation. Hard to do at LF, so in practical terms
> the advice is usually to pretend there is a "rubber sheet" pulled over
> your property and make sure the whip is the highest point.

I'm trying to figure out the basis for that "theory".

The fields from distant sources are electromagnetic, and that is 
actually a different mechanism than electric and magnetic 
induction  fields. While the impedance of an active antenna near 
the antenna is high.....making it more responsive to electric 
induction fields near the antenna.....the arriving distant wave cares 
less!

It is standard practice to calibrate small loop antennas in mV per 
meter, even though they are low impedance antennas and not 
considered "voltage probes". The calibration works just fine as long 
as we are in the farfield with the source. We could just as easily 
calibrate a "voltage probe" in amperes per meter.

As a matter of fact, if we look at field impedance of small antennas 
a small "magnetic loop" actually is more responsive to the electric 
field at distances of about 1/10th wavelength or larger while a small 
 "voltage probe" does the opposite, and is more responsive to 
magnetic induction fields. At about 1/2 wl the field impedance of 
both becomes essentially identical, and remains identical at all 
distances further than that. It is only very close to the antenna that  
field impedance is high for probe and low for a loop, and all work on 
the same principles. 

Since that is true, height makes no difference except for pattern 
and sensitivity considerations. That's mostly true because it is 
virtually impossible to decouple the small whip from whatever it is 
mounted on, so we are in effect building a taller antenna when we 
mount a voltage probe system on top of a mast. Even if we use a 
insulated mast, the feedline will remain part of the antenna. Think 
of it as a top-fed vertical!

My short 160 meter verticals work just fine at 1 or 2 feet of height, if 
amplified with a JFET. They hear everything a taller vertical will 
hear, even though they certainly are not anywhere near the tallest 
thing around. They couple no worse or less to other antennas than 
any other system I have, although they are nearly impossible to 
isolate from the feedlines and mounting masts unless ground 
mounted.
 
> My whip is mounted currently on a metal mast, with the last six feet
> being 1/2" PVC with a wooden dowel inserted for strength. The feedline
> is isolated using a torroid. Currently the whip does not use a
> counterpoise at all, though I plan to install a virgin ground at the
> base of the mast soon to see if that improves performance.

Is this an isolated primary-secondary type of transformer? If so, 
how do you get voltage past it?

I don't see how it is possible to obtain enough impedance to isolate 
the whip from the mast, unless we have a symmetrical elevated 
counterpoise that has a much lower impedance than the mast just 
below the whip and an isolating impedance that is many times 
larger than the common mode impedance of that elevated 
groundplane below that point.

Not only does this system require a**very**  high choking or 
isolating impedance, the sign of the impedance has to be correct. If 
we attempt to isolate a small whip system from a mast or feedline 
with an *inductive* choke or isolation system, the choke actually 
increases common mode currents and brings the mast and 
feedline MORE into the system!

You can verify this on Eznec and other modeling software, or in 
practical tests as I have done. 

I just modelled an "active whip" on Eznec. Of course you model 
receiving antennas as a source.

With no feedline isolation, the feedpoint at 38 feet and a two-foot 
whip, common mode current ten feet off the ground in the feedline 
is 26% of the feedpoint current. When I add a 10,000 ohm 
reactance choke at 36 feet in series with the feedline, current at 
ten feet is 29% of the feedpoint current! Woops, it goes up and not 
down!

When I make that choke 10,000 ohms capacitive reactance, the 
feedline current at ten feet is 24% of the feedpoint current! 

Since all antennas respond to the equivalent of ampere-feet, we 
haven't changed much with an isolation device!  

Just as I would expect in theory and as I have actually measured 
here, Eznec confirms that it is virtually impossible to make the 
feedline and mast NOT part of the radiator when using a small whip 
antenna.

When we elevate the probe, we are really just making a taller 
vertical. The exception would be if we had a large elevated 
groundplane that had much lower common-mode impedance than 
the mast and the choking system.

Perhaps the rubber sheet theory appeared as an attempt to explain 
why the response of the antenna changed when we simply moved 
what we thought was the antenna higher, because we just didn't 
realize the antenna was actually getting larger and NOT just being 
moved higher! Perhaps the fact the feedline and mast was a major 
part of the system was never considered. 

> I'd love to hear more about your experience with active whips on 160
> Meters. Mine is tuned active whip, using a litz wire inductor,
> designed by Bill Bowers. Very sharp "Q" which makes it perfect for LF
> work, but perhaps a bit too sharp for work on 160 meters.

My probes were all ground mounted, but now I have replaced them 
with a hat-loaded vertical system. All were worked against a 
reasonable ground system (lower ground impedance than the 
antenna element's common mode impedance), since it is virtually 
impossible to isolate them from common mode currents otherwise.

None were tuned, or if they are tuned they are resistor loaded to de-
Q them since they are part of a phased array. You can see my 160 
verticals at www.w8ji.com under small vertical arrays. One array is 
a 350 foot circle of 8 verticals, and the others are end-fire broadside 
arrays fired in specific directions.

Presently most of my vertical antennas are about 20 feet high, 
although I have used shorter antennas. Antennas as short as one 
foot will get me into propagated noise in my very quiet rural 
location, but then they need to be amplified. It takes less active 
hardware to use a taller element, that is the sole reason I use a 
modest height element.

Making the elements taller increases signal and noise in the same 
proportion, and does not change how the elements couple to other 
objects one bit. It will change things of course if the design is 
flawed, and the feedlines are not isolated!
73, Tom W8JI
W8JI@contesting.com