Radio Habana Cuba
Dxers Unlimited
Dxers Unlimited's weekend edition for 22-23 October 2005
By Arnie Coro
radio amateur CO2KK
Hi amigos radioaficionados ! You are welcome to the weekend edition of your
favorite radio hobby program. Dxers Unlimited , Radio Havana Cuba's twice
weekly program devoted to the promotion and development of our wonderful
passtime... RADIO ! A debate is now going on among solar physicists ... some
of them believe that solar cycle 23 has already ended, while others consider
that the recent upsurge in solar activity is something quite normal, and
that the next several solar rotations will confirm their point of view, when
they expect that the numbers of sunspots will go down to a consistent
minimum lasting no less than 6 solar 27 day rotations... In the meantime
here is my advice... keep monitoring the daily solar numbers, write them
down carefully and act accordingly... in other words, if you see that solar
flux starts moving up for several days, then watch its effects on the
behavior of the F2 layer maximum
useable frequency curve.... Using this information plan your amateur radio
or short wave listening activities to make the best possible use of
propagation windows. By the way, there is a really excellent, easy to use HF
propagation software ... it is also rather compact and produces excellent
results by feeding simple data to it... W6EL, that's W6 Echo Lima, Sheldon
Shallon, an American radio amateur has made this software freely available
to radio hobbysts worldwide... You may donwload W6EL's propagation
forecasting software from several Internet sites, but in case you don't find
it, just send me an e-mail and I will more than happy to e-mail the dot zip
file to you via e-mail... The program is very easy to install and learning
how to use it is also very intuitive... This is freeware amigos, freeware
for amateur radio and short wave listeners around the world, that want to
optimize their operating schedules for maximum Dx possibilities, or for
other purposes, like holding a schedule with another ham...Again , if you
don't find W6EL's propagation software after running an Internet search
engine scan... then drop me an e-mail to arnie@rhc.cu, again arnie@rhc.cu
and wait for my reply with the attachment... More radio hobby related items
will follow in a few seconds after a short interval and station ID... I am
Arnie Coro in Havana.
Si amigos, you are listening to Radio Havana Cuba, the name of the show is
Dxers Unlimited, and here is our next item , the information about the TTFD
wideband antenna design that I have made available for many years at several
Internet websites continues to attract the attention of radio hobby
enthusiasts worldwide, and YES, I do get a lot of e-mail messages and
letters asking questions or commenting about the TTFD's unique
characteristics... Among the most recent ones, a US radio amateur tells me
about his experience with a VHF bands TTFD that he built some time ago, but
that unfortunately was lost to bad weather. His letter started by pointing
out that my advice to would be builders of the TTFD not to use wirewound
resistors for the terminating resistance of the TTFD was wrong... and he
added that there are wirewound resistors built in such a way as to be
non-inductive, in other words that a resistor made of high resistance per
unit length wire, could be configured so that its inductance will really
very small, something achived by winding the wire in two sections that will
oppose to each other, effectively reducing the inductance to a very low
value. According to measurements made by Tom,the sender of the e-mail, the
inductance of those special resistors is really very low, making it possible
to use them as non-inductive terminating resistors for TTFD or Tilted
Terminated Folded Dipole antennas. Here is now , and I am going to quote
from his e-mail what amigo Tom told me:
Hello Arnie.
I found myself working on antenna projects here (WHY do I always get
interested in antenna work just before the snow season starts here?), and my
favorite antenna, the T2FD is in the center of my plans.
Going over the usual information on the internet to see if there is anything
new on them out there, I ran into your page on T2FDs.
You, like everyone else, emphasize that the terminating resistor MUST be
noninductive, but you made an error in your advice.
You say that the resistors MUST NOT be wire wound. Did you know that there
ARE wire wound, noninductive resistors out there?
The Sprague Company made a series of them, mostly high wattage (100 watts or
more), and some of them are made for extremely high voltage use (sealed in
glass tubes). I've found them to be PERFECT for use in T2FD antennas made
for transmitting, because they can dissipate a reasonable amount of power.
I found a number of different values of them at hamfests, and did a bit of
experimenting.
On an RF impedance bridge I found that they ARE INDEED pretty much non
inductive. To find out how this is possible I took two of them apart to
examine thier construction.
A glass sealed one for 500 ohms was found to be two 1000 ohm wire wound
elements connected in parallel. They were wound in opposite directions on
ceramic tubes, and one was placed inside of the other so that the inductive
fields of each element cancelled each other out!
A ceramic unit of 50 ohms was broken open, and it was found that the
resistor winding kept reversing winding direction along the whole length of
the form, again cancelling the inductive field out the length of the
resistor.
Identifying these resistors from regular wire wound ones is easy. In the
information printed on the body of the ceramic units, one ine of information
indicates the wattage, with the letters "N I" printed after it. For example:

SPRAGUE
KOOL OHM
50W NI
475E5105
The glass body units I have use brass end caps that snap into clips for the
electrical connections. On one end cap is stamped the resistance, followed
by the letters "N. I." . Sometimes the same kind of information is marked on
the resistor's glass tube body, similar to the markings on the ceramic body
resistors.
I've been able to find these noninductive, high wattage resistors very
easily at hamfests, and since the sellers usually don't know they're
noninductive, or even grasp the significance of that, they usually go quite
cheaply. The last one I found was a 500 ohm, 200 watt glass unit that was
purchased for fifty cents!
I don't know if Sprague still makes these resistors (the latest date code I
ve found is 1967); I haven't been to thier web site to check. I hope this
information is useful to you.
Tom
Well that was the first part of our really interesting exchange with amigo
Tom, W9LBB...
After reading his e-mail I sent him my points of view .... and again, now I
quote from the text of my answer, listen carefully please
Dear amigo Tom: Thank you very much for your information about the non
inductive wire wound resistors... I have worked with such resistors for a
long time, and the reason I didn't mention them in my articles about the
TTFD antenna is that I thought that they would be very difficult to find to
the average homebrewer , especially those that are not radio amateurs, but
only short wave listeners, and hence do not attend hamfest's fleamarkets !
For receive only purposes, the typical 2 Watt carbon resistors of high
quality are ideal, especially if you put several of them in pararell in
order to cancel as much as possible any "residual" inductance... (www.cebik
com <http://www.cebik.com> ) has experimented a lot with a "new" version of
the TTFD, using a higher value of terminating resistors than previous
designs ( near 900 to 1000 ohms ) Yaesu is selling very succesfully the YS30
broadband antenna, that uses a non inductive 100 Watt resistor... By the way
I have used the TTFD design for an excellent VHF antenna for monitoring
Sporadic E openings... It provides very good reception, almost
omnidirectional from 30 to 100 megaHertz... so it is ideal to use with a
scanner programmed to pick up only the frequencies of VHF Lo Band stations
between 30 and 50 mHz that provide a nice clue about possible 6 meter band
openings... 73 and DX YOur amigo in Havana Arnie Coro CO2KK Host of Dxers
Unlimited And the exchange continued when Tom answered back to me saying:
Hello Arnie!
Yes, your reasoning for not mentioning these resistors in your article makes
a great deal of sense; in addition to the problem of availability for some
readers, you also have to keep things somewhat simple because experience and
expertise levels of your readers are all over the map, from grade school
beginner to professional engineer. A lot of folks might have problems
dealing with the concept of inductive versus noninductive resistors. I've
been working with T2FD antennas for about 12 years now, and I too came up
with one for VHF work; I built one out of copper plumbing pipe, sweat
soldered together, and mounted as a vertical. I cut it for a range of 30 -
150 MHz and it was VERY successful until it was destroyed in a bad windstorm
I plan to put another one up in the future; while it is (in my opinion)
somewhat inferior to a WELL DESIGNED discone (NOT those little things that
Icom and Radio Shack are selling!!!), it's much simpler to construct, a lot
more sturdy, and takes up much less space on a roof or tower than a discone
does.
I've seen the Cebik analysis of the T2FD and found it VERY interesting! His
two test models, especially the short, narrow antenna run with a higher than
usual terminating resistance caught my attention, and is the direction I'm
working in now. His disregarding the traditional wire spacing on it gave me
an idea here. Since the T2FD is a nonresonant antenna, just about everything
in it's design is extremely flexible and tolerant to change. Taking that a
step further, my current experiment is to construct a T2FD out of 450 ohm
ladder line"!
One of my first antennas was cut for 1500 - 7500 KHz, and using the World
Radio TV Handbook formulas, the end spreader was 6.5 feet of oak 2" x 4"! I
got this monster up all right (it was 220 feet long!), but the compression
forces on the end spreaders were TREMENDOUS; every six months there would be
a loud CRACK! and a spreader would break. BTW, I hung it as a dipole, not as
a sloper. It was at about 30 feet up, strung between two trees. I fed it
with 450 ohm ladder line, with a homebrewed 9:1 balun on the ground to match
50 ohm coax. For all the problems, this was the best antenna I've ever had
for the range below 4 MHz. It's performance was AMAZING...In an upcoming
edition of Dxers Unlimited, I will continue to read to you more of the
comment made by amigo Tom about the TTFD antenna designs...
And now , just before going QRT here, as usual, our HF plus low band VHF
propagation update and forecast....in an easy to understand form, so that
you can make good use of it as soon as the program is over... Solar flux
hovering at extremely low levels, the A index is only ONE !!! And the number
of sunspots is at 15...Expect nice DX on the AM broadcast band for the next
two to three days. Daytime MUF should be peaking best at around 23 to 25 mHz
but 10 meters amateur band openings are also possible...
Hope to have you all listening to the mid week edition of the program next
Tuesday and Wednesday UTC days... and don't forget that my direct e-mail
address arnie@rhc.cu, is always ready to receive your signal reports,
comments about the program and radio hobby related questions amigos !!!