RADIO HABANA CUBA

HOW TO BUILD THE TTFD-2:
A BROADBAND ANTENNA FOR
SHORTWAVE RECEPTION AND TRANSMISSION

The lowest-cost approach for home brewing
broadband, general-purpose antenna for shortwaves

by Arnie Coro, Dxers Unlimited
Send your comments, questions and ideas directly to me at: arnie@radiohc.org


During the past few years I have built many TTFD broadband antennas. The Tilted-Terminated-Folded-Dipole can be designed for frequencies between 1.5 and 150 mHz. The original prototype was the brainchild of an amateur radio operator W3HH, and the antenna was used first for professional communications systems. For all practical purposes, the TTFD provides excellent bandwidth on at least a 3 to 1 ratio... i.e. a TTFD designed for a low frequency cutoff of 7 megahertz, will provide excellent performance from slightly below its theoretical cutoff frequency - in this case about 6 megahertz, to no less than 21 megahertz.... although practical installations show that the upper frequency limit extends to around 30 megahertz or so... giving an effective 4 to 1 frequency ratio. My TTFD designs are for a low frequency cutoff of about 3 megahertz, which is a BIG ANTENNA, an intermediate length version with a cutoff around 7 megahertz and a compact version with a low frequency cutoff of 14 mhz.

I also designed the VBW-1, the VHF Broadband Wonder, a modified TTFD for 25 to 150 mHz operation. (See my article in October 1997 CQ-VHF magazine.) For general SWLing... (shortwave listening) I'll recommend the 7 mhz to 28 mHz version (which in practice is a good performer from 6 to 30 mHz). Those of you with restricted space may obtain nice results with the more compact TTFD-3 with a low frequency cutoff of 14 megahertz. I used one of this TTFD-3 antennas for monitoring the SWL bands from 19 to 13 meters, and operating on the amateur 20,17,15,12 and 10 meter bands... plus 11 meters CB, with excellent results.

What to expect and what not to expect from a TTFD

The TTFD is a broadband antenna... it is a very quiet antenna indeed, as it picks up very little man made noise as compared with a vertical or even a horizontal dipole. The SWR, Standing Wave Ratio, when transmitting may vary from an almost perfect match of 1.1 to 1 at some frequencies to a rather poor 3.0 to 1 or even 4.0 to 1 at other frequencies. This is why I always recommend using the TTFD with a wide band antenna tuner If you compare the TTFD with a dipole cut for a certain specific frequency, three things may happen:

In real-life installations, cases 1 and 2 are much more frequent, with case 3 only happening at the higher end of the antenna's coverage.


TTFD Antenna Diagram

Fig 1. Properly installed, TTFD is omnidirectional over most of its operating range.

So, you will ask me, "Arnie Coro, my friend, why build a TTFD?" OK, here is the answer: Because you and I cannot own an antenna farm with a collection of rhombics, log-periodics, wideband dipoles and curtain arrays! Building your own TTFD will be like having a collection of dipole antennas for many frequencies, while having to install a single antenna and feedline.

It's not perfect... It will show some loss at some frequencies, but it will work very well, especially if you use the TTFD with a nice antenna tuner.

HOW TO BUILD THE 7 to 30 mHz TTFD-2 ANTENNA

Yes, there is a formula for the TTFD overall length:

100/ freq in megahertz = antenna length so 100/7 = 14.28 meters... for all practical purposes = 14 meters... for those of you still without a METRIC mind... 14 X 3.28 = 45.9 feet, for all practical purposes 46 feet...

So that's the overall length of the TTFD-2

Now, the separation between the lower and upper parts of the antenna, which looks very much like a folded dipole.

Here is the formula:   3/ freq mHz

So 3/7 = 42.8 centimeters.

For all practical purposes it's 43 centimeters... this is not critical; if you wish you can make the separation any value between 40 and 50 centimeters and performance will be the same. Now for the all-important TERMINATING RESISTOR. If your plans are for an R/O or receive only antenna, then my advice is to use two 680 ohm 2 watt carbon resistors connected in parallel to make a 390 ohm 4 watt resistor. This resistor should be completely protected from the weather, enclosed in a sealed container.

PERFECTION CALLS FOR A RARE 6 TO 1 BALUN TRANSFORMER

Yes, in order to obtain the most perfect match over the wide frequency range, the use of a 6 to 1 balun is recommended... But my friends, TRY TO FIND ONE ALREADY MADE!!! NO, you won't find it... so you are faced with two options:

If you go for 1... good luck! It's nice to learn more about Baluns and there is an excellent book by Jerry Sevick (Dr. Sevick, Ph.D.) that will guide you into learning a lot about balun and ununs (ununs are another kind of transformer built using ferrite cores).

If you choose the no. 2 approach, that's OK, you will have to tweak the antenna tuner controls a little more.

REMEMBER... the TTFD looks very much like a FOLDED DIPOLE, except it is broken at the center of the upper portion to insert the loading resistor, and at the center of the lower portion to connect the BALUN (balanced side).

The antenna is fed from 75 ohm coaxial cable (any length). It should be installed TILTED... that is, with an inclination of no less than 20 degrees, with the ideal TILT at 45 degrees. This requires just a single tall mast, for the upper end.

The 7 to 30 mHz version can be installed using a small 2 to 3 meter high mast at the lower end. If you have the space for the 7 to 30 mHz antenna, by all means build it and install it... You will find that it performs very well even when receiving stations on the 6 mHz or 49 meter band. If you are really very short of space, then build the 14 to 56 mHz version; it will still provide useful reception down to 11 megahertz or so.

Do remember that the TTFD is a COMPROMISE ANTENNA... Don't ask this antenna for the performance of a 14 element LOG PERIODIC at 30 meters above ground!!!

If you build and install a TTFD, I am sure that you are going to keep using it, even if you may install much better antennas in the future. The extreme convenience of a single skywire, with a single feedline that can let you work over such a tremendous bandwidth is the feature for which the Tilted Terminated Folded Dipole is now so popular among professional users of the shortwave spectrum.

More theory

Yes... the TTFD -- Tilted Terminated Folded Dipole -- is a relative of another famous antenna... the RHOMBIC, known among experts as the "Queen of Antennas" for its extraordinary performance and reproducibility of its radiation patterns. A TTFD is "terminated" like a full rhombic, that means that a NON-INDUCTIVE RESISTOR is placed at the end of antenna, something which provides a LOAD or TERMINATION to the travelling wave propagating along the antenna.

But, the big differences between the TTFD and the RHOMBIC, are that the first is much smaller, has little or no directivity and fits into a rather small lot, while a RHOMBIC antenna for covering 6 to 12 megahertz may be several city blocks long, andexhibits a very narrow horizontal radiation pattern (directivity). Nevertheless, the TTFD is a very practical broadband antenna when all its limitations are fully understood.

Mechanical considerations

Building a TTFD for the 7 to 30 mHz frequency range requires taking into account some mechanical design considerations... For example, you can't use a very small diameter wire for the antenna, as its span is such, that at least no.16, and better yet no 14 or even no 12 wire is needed.

The upper and lower wires of the TTFD must be kept at a uniform distance, something that is achieved by the use of several fiberglass or PVC plumbers pipe spreaders.

I have used even broomsticks for the spreaders of an emergency TTFD... but I won't recommend wood without very extensive weather treatment.

Fiberglass spreaders, of about 60 cm or 2 feet in length are ideal for the TTFD-2 7 to 30 mHz antenna. You can hold the wires to the spreaders in several different ways... but the one I prefer is NOT to drill a hole to the spreader,so that its mechanical resistance won't be affected.

For the length of each arm of the TTFD-2, I use no less than 3 spreaders... in my case I make them from plastic PVC plumbers pipe which is the locally available material.

The ends of each side of the antenna can be made easily. I use aluminium T or I beams of about 25 X 25 mm (1 x 1 inch) and in this case I do drill a small hole at each end of the 60 cm aluminium beam to let the wire go trough.

I always make the TTFD antennas as two continuos wires... One on each side... Start at the bottom insulator, were you connect the feedline, proceed to the end , thread the wire trough the bottom hole of the aluminium spreader... go up, then thread the wire trough the upper hole of the spreader and proceed to the upper insulator, the one that holds the terminating resistor.

The antenna requires either 4 or 6 insulating spreaders, and the two aluminium spreaders at the ends.

How to support the terminating resistor.

The terminating resistor (made of two 680 ohm 2 watt carbon non inductive resistors properly sealed against the weather) is connected across the antenna's upper insulator.

With this 390 ohm 4 watt maximum dissipation resistor, the antenna will present an impedance very near 450 ohms.

It will also show nice performance not only when receiving, but also when transmitting with a maximum power of 10 watts on the AM mode, about 15 watts on CW and about 20 watts on SSB. But, I prefer to consider the antenna as capable of transmitting with a maximum power input to it of 10 watts on any mode, to simplify things a bit.

If you need to use the antenna with higher power... then you must prepare a special series pararell combination of non inductive carbon resistors capable of dissipating around 33 percent (one third) of the power used by the transmitter.

There are many possible combinations to reach values between 350 and 400 ohms, which all seem to work quite well when using the antenna with a 6 to 1 (the best approach) or a 4 to 1 balun (a compromise).

The TTFD should be ALWAYS USED WITH AN ANTENNA TUNER... For two very good reasons:

1. It's a wideband, broadband antenna, and energy supplied to it at any frequency within its operating range will be RADIATED... something YOU and YOUR FELLOW USERS of the shortwave spectrum won't like to happen.

2. The antenna does not provide a perfect 1:1 Standing Wave Ratio at all operating frequencies.

For receive-only applications, you can get away without using the antenna tuner, but TRY IT, with and without a tuner, and you will soon join the ranks of all of us that have an antenna tuner tied to our radios permanently.

Don't Use the TTFD as an Inverted V

One final bit of advice: Never use the TTFD in an inverted configuration.It will work on receive, but on transmit you will get a very distorted radiation pattern...

Any doubts?

Just click on the mail-to facility right here, and send me an e-mail with your questions...

But before you finish reading, let me tell you that this, the TTFD, is the most practical general purpose shortwave receiving antenna I have ever used... And for transmitting, via an antenna tuner, it happens to be able to radiate a signal on any frequency withing its very wide range.

More about this wideband, easy to build and to keep up general purpose antenna:

High-power terminating resistors:

If you ever plan to use the TTFD for running more than 10 watts, you cannot do so with the terminating resistor used for receiving and low power transmitting.

You will still need a NON-INDUCTIVE, i.e., not WIREWOUND resistor of around 390 to 410 ohms ... but this resistor must now be able to DISSIPATE no less than 33 percent of the transmitter's power at some frequencies.

During my years of playing around with TTFD's, I have found that at some frequencies the terminating resistor (or resistors, when I had to assemble a series-parallel combination of them) would run pretty cool, while at some other specific frequencies the resistors would get hot.

OK, this I knew because I ran 10 minute tests with full carrier on (and a proper station ID of course) then lowered the antenna via the pulleys at the top of the higher mast, and then took a very fast temperature sample of the terminating resistors.

So, if you do want to have a TTFD capable of running at the typical 100 watt HF amateur transceiver power level, here is how to do it: Assume that you will need a terminating resistor capable of dissipating 33 watts... for all practical purposes assume from 35 to 50 watts to play it safe.

Then, you have two options:

Remember you are looking for a value near 400 ohms... and that you are going to play with standard value 2 watt resistors. The 40 watt dissipation unit will require no less than 20 resistors while a 50 watt unit will require 26, which will then be rated at 52 watts. BUT... BEWARE: You don't want to run those resistors at full theoretical dissipation, so you must DE-RATE THEM. This is done in the following way:

You will call your 2 watt carbon units... 1.3 watt resistors, or even better, deal with them as 1 watt units. Now, this will require many more resistors, but the antenna will be able to work even with an AM rig, full 100 percent carrier and 100 percent modulation!!!

How to make your 400 ohm terminating resistor:

I follow this procedure, which after many experiments proved to be the right way of doing it. For the 400 ohms resistance, I assemble TWO sets of resistors that will give me 200 ohms each, and then wire up both sets in series to reach 400 ohms; ten resistors of 2000 ohms and 2 watt dissipation, connected in parallel, will provide a 200-ohm, 20-watt resistor.

Two such assemblies connected in series will provide the required 400 ohms. BUT, NOT AT 40 WATTS... remember that we decided to call the 2-watt resistor a 1.33 watt resistor to be on the safe side. Well... we have now a 400-ohm resistor made of 20 X 1.33 watt units, which is 26.6 watts... for all practical purposes, 25 watts.

This assembly will work very well with a 100-watt rig on the CW and SSB modes, but will may run HOT on some frequencies if you operate on AM or RTTY modes.

Anyway... the 20 resistors of 2 ohms each termination is an excellent choice for most amateur stations. You must put that assembly into a nice weather protection, so that humidity will not penetrate. One of my TTFD's termination lasted several years enclosed into a plastic refrigerator type food box...

How to look for the resistors:

Hamfests... fleamarkets at hamfests are excellent sources of components for your electronic and amateur radio experiments. Sometimes you can find high quality carbon resistors in large quantities at bargain prices, as they may come from manufacturers production overruns. Be really sure that the resistors ARE NOT WIREWOUND!

One final comment... a TTFD supported from a tall mast and sloping at 30 to 45 degrees, is practically omnidirectional, that is it will receive and transmit equally well in all directions.

The antenna's polarization is a mix of both horizontal and vertical, something that may actually prove to be a bonus, as it seems to provide some sort of anti-fading properties.

I hope that this INFO-PACKAGE about the TTFD is helpful. If you feel like you need to ask something more, JUST CLICK THE MAIL-TO feature and send me an e-mail. Yes, I will be very happy to answer your questions my friends!!! And once you finish building your TTFD and after some trials and tests... also tell me and other DXERS UNLIMITED listeners!

Arnie Coro
Host of "Dxers Unlimited"

Postal address:
Dxers Unlimited
Radio Havana Cuba
PO Box 6240
Havana, CUBA   10600

Send e-mail to Arnie Coro


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