Re: Building a T2FD antenna

David G. Nagel wrote:
Rick (W-A-one-R-K-T) wrote:

I've pretty much decided to go ahead and build a T2FD "radiating dummy
load", hopefully for use from 2 to 24 MHz.

This will be almost (but not quite) exclusively for receiving, wherein my
limiting factor is usually noise anyway, rather than antenna efficiency. If somebody appears on one of the frequencies I scan that I want to talk
to, if he's not S9 or better I'll likely switch over to a "real" antenna. But, the occasional use for transmitting means I'll need the appropriate
power resistor on the load end (100 watts or so).

You guys have suggested googling for "T2FD" and "TTFD" and I have done
that, but I haven't yet found anything that really lays out how to design
and build one of these things, what the formulas are, etc.

Some of the Google hits suggest using baluns and resistor values all the
way from 11:1 balun, 550 ohm resistor, and 50-ohm coax to 4:1 balun,
300-ohm resistor, and 75-ohm coax. Some seem to imply that one should use
a resistor value equal to the coax impedance times the balun ratio, and
others say that the resistor value should be some percent higher than the
coax impedance times the balun ratio (one site suggested 75 ohm coax, 4:1
balun, and 390-ohm resistor).

Obviously I'd like to do this a cheaply as possible and I do have a W2AU
4:1 balun sitting on the shelf here, so would 50 or 75 ohm coax, 4:1
balun, and anywhere from 300 to 390 ohms for the terminating resistor work
as well as a higher resistance value and a higher ratio, scarce, and more
expensive balun?

I have seen formulas, which I can no longer find, that say what the length
should be for a given minimum frequency, and what the spacing between the
conductors should be. I assume that the spacing of the conductors has a
lot to do with the balun ratio and terminating resistor.

I guess if I can find the length, conductor spacing, and terminating
resistor value needed for 2 MHz minimum and 4:1 W2AU balun I should be all
set.

Thanks...


The terminating resistor of a B&W antenna is 600 ohms resistive i.e. there is a 600 ohm resistor connected to the ends of the folded dipole antenna. It's purpose is to swamp the current leftover when the standing wave is formed. The balun is of necessity a 12 to 1 ratio balun. Other values may be used to other effects.

I don't know that the balun ratio corresponds to the resistor value necessarily, although it's true that B&W's patent essentially says that. One might spend a few minutes modeling various combinations to find out.

Looking at W4RNL's page:
http://www.cebik.com/wire/t2fd.html
says
"(The general recommendation is to use a terminating resistor that is about 5% to 10% higher in value than the feedline characteristic impedance.)"

One might also look at B&W's patent (4,423,423)
http://patft.uspto.gov/netahtml/PTO/srchnum.htm

"The non-inductive resistive network 78 is preferably a single wire wound resistor having an impedance dependent upon the selected frequency range of the antenna. This impedence must be totally non-inductive for the antenna to function properly. The resistive network 78 must also have a power dissipation capability which should be at least one quarter of the power transferred through the antenna matching means 46 to the antenna from the radio frequency generator. Of course, in order that the environment not affect the operation of the resistive network 78 the casing 80 should be weather proof.

The antenna balancing or load impedance is chosen to give the antenna its broad band characteristics while retaining its shorter overall length. It has been found that a load impedance in the range of 100-900 ohms will work satisfactorily with an antenna having a frequency range of 3.5-30 MHz and an overall length between 50 and 130 feet. A load impedance of 600 ohms is preferred because it increases the bandwidth of the antenna while decreasing the voltage standing wave ratio associated with each frequency in the 3.5-30 MHz range. Since the output impedance of the radio frequency generator is most often 50 ohms, the ratio between the two coils 74, 76 of the antenna matching means 46 will be 12:1. The antenna load impedance of 600 ohms works equally well with a frequency range of 7.0-30 MHz. However, for bandwidths which include lower frequencies, such as 1.8 MHz, and for other frequency ranges of smaller bandwidths, other impedances in the range of 100-900 ohms may be preferred in order to decrease the voltage standing wave ratio for those frequencies while permitting transmission and reception over a continuous bandwidth. With the use of a smaller impedance in the antenna balancing means 44, the ratio of the coils 74, 76 in the antenna matching means 46 and the length of the antenna will vary to reflect such changes.
"

"An antenna balancing or load impedance of 800 ohms has been found to cause the antenna 90 to perform quite well where said antenna has a frequency range of 1.8-22 MHz and an overall length averaging 185 feet. The 800-ohm load impedance is preferred because it increases the bandwidth of the antenna while decreasing the voltage standing wave ratio associated with each frequency in the 1.8-22 MHz range. This impedance increases the ratio between the antenna matching means and output impedance of the radio frequency generator to 16:1.
"

The biggest problem is the balun. I haven't seen any specs for making one. The resistors need to be carbon non inductive high wattage.

Or those nice inexpensive Caddock non-inductive resistors available from Mouser for about $2 each for the low power versions, up to $10 each for the higher power ones.

http://www.mouser.com/catalog/631/552.pdf

http://www.caddock.com/Online_catalog/Mrktg_Lit/MP9000_Series.pdf


available up to 100W rating (assuming bolted to an appropriate heatsink). I'd string together a bunch of an appropriate resistance, either series or parallel, your preference.

Let us know how you do.

Dave WD9BDZ
.

Relevant Pages

  • Re: Phase splitter with capacitance load.
    ... have the same output voltage, EL, as they stand, if a further load is ... The IEEE Dictionary of electrical terms defines impedance, ... Consider the following resistor network. ... Now let's apply some additional load at nodes 2 ...
    (rec.audio.tubes)
  • Re: MFJ-269 Antenna Analyzer experience
    ... the indicated impedance at the TX end of the feeder is actually the antenna feedpoint impedance. ... Somewhere in the future, as hams get more familiar with this kind of thing, I see folks installing a little relay box at the feedpoint of the antenna that has a short, open, and load, as well as the antenna. ... I hook the the feedline on port 1, the antenna on port 5, the short on port 2 and the load on port 3. ...
    (rec.radio.amateur.antenna)
  • Re: Whip antennas with coils
    ... Does the use of "output impedance" here mean that he transmitter can be ... that a transmitter rated for a nominal 50 ohm load has a source impedance ... Using your antenna as an example, ...
    (rec.radio.amateur.antenna)
  • Re: correction to question about wire antenna and tuner
    ... replaced the antenna with a dummy load of the same impedance (a resistor ... This is when max power transfer to the load occurs. ...
    (rec.radio.amateur.antenna)
  • Re: question about wire antenna and tuner
    ... replaced the antenna with a dummy load of the same impedance (a resistor ... Taking the derivative of Pload with respect to Zload and solving the max/min ... This is when max power transfer to the load occurs. ...
    (rec.radio.amateur.antenna)