In my previous post I tried to elaborate on the ‘magical properties’ of the
folded counterpoise (FCP).
My first conclusion is/was there is no such thing as ‘current cancellation’
in a counterpoise. Fields cancel, not the current, otherwise
the counterpoise never can act as a counterpoise.
I built an inv-L version for 40m in Germany and was forced to
conclude that it performed overall better than a 11m high inverted-V.
Secondly, there is no need for an ‘isolation transformer’ or other fuzzy materials.
A FCP based antenna can be made of simple (yes, also insulated!) wire.
The trick (as always!) is to build an antenna from a defined starting point and
in this case make the FCP-antenna resonant ( j = X = 0 ). From there the antenna
may be tweaked to the desired impedance ( Z = 50 + j0 Ω), if necessary.
My initial observations: despite all discussions a FCP as counterpoise seems reasonable,
i.e. not too good, but also not too bad. Especially concerning the FCP footprint.
Bear in mind, the radiation resistance (Rrad) of a 1/4λ (‘full size’) vertical above
ideal GND is around 36Ω. So if you measure an impedance of 50Ω there have to be
(earth + construction) losses involved.
Measuring VSWR doesn’t say anything, except that your transmitter feels it’s pushing
all his energy into the antenna contraption. Whether all this energy will be converted into
electromagnetic / radiated energy is the heart of the matter.
From the transmitters perspective a 50Ω dummy load is the ideal ‘antenna’!
However, this ‘antenna’ does not radiate (if everything is well ; -).
But how ‘good’ or how ‘bad’ is a FCP antenna, from a slightly more quantitative perspective?
The proof of the pudding is in the eating!
So, I built a new FCP with full size radiator (1/4λ) and compared it with
an antenna that is generally considered to be ‘reasonable’ and beyond suspicion:
A full size (1/4λ) vertical with two 1/4λ elevated sloping radials, defined as the ‘reference vertical’.
I made a movie of the experiment, see below. The aggregated RBN data of the experiment is here.
I’m not a super spreadsheet wizard, below is one of the RBN RX sites with perceived SNR’s.
Distance is ca. 1900 km. (Click on image to enlarge in a new tab)
A more detailed analysis of the RBN data will be published soon.
Measured relative field strengths in the movie have to be corrected for the distance.
This can be done easily because distances between measuring points and antennas
are depicted in the movie.
After correction my calculations reveal that the FCP (PA2FCP) and reference antenna (PA3REF) generate
almost equal field strengths at the two measurement locations.
The deltaloop seems an outlier, however its radiation pattern (‘egg shape’) has to be considered.
Update: I received quite a few emails on how I constructed the 40m FCP vertical in the movie.
With a ground drill (diam. ca 8cm) a 40cm deep hole was drilled.
One of my 12.5m long fibreglass poles was inserted in this hole.
However, it’s also possible to mount the pole to a caravan adze ; -)
The modeled dimensions were reduced with the velocity factor (vf)
of the used wire. Bases on my experience and measurements the vf of my wire was 0.95.
My wire: PE insulated, 3mm OD, ca. 1.75mm ID copper.
In total I needed (1067 * 0.95) + [ 5 * (279 * 0.95)] + ( 2 * 10 ) = 2360cm wire.
(see this NEC file or picture below)
Spacer material was ‘electricity’ PVC tube, 88cm total, divided in three (3) pieces
of 22cm, and two (2) pieces of 11cm. Thus, a spacer every 132.5cm
Holes for the wire were drilled somewhat skewed so that the spacers fixate themselves
when the FCP is ‘stretched’.
FCP ends were connected with rope to bamboo poles, which were guyed with
rope to strengthen and fixate the contraption.
After building the contraption accordingly my MFJ-269 measured resonance (j = X = 0)
around 6.9 MHz. R was around 52Ω. At 7020 kHz Z was 55 + j15 Ω (iirc).
The FCP end was shortened with 15-20 cm and a (relatively broad) X = 0 dip was measured
at 7020 kHz with R = 54Ω. Good enough.
When (at resonance) R is too high, increase the FCP height. However, the contraption
has to be trimmed back to resonance by either tweaking the vertical part
( = ‘nuisance’ because the pole has to be dismounted) or the FCP end.
This may require some iterations.
The feedline contained a CM choke of 10 μH, consisting of 5 TDK clamps of 2 μH each, XL @ 7MHz = 440 Ω.
Below some pictures to visualize the aforementioned. (Click on each to enlarge in a new tab)