New T2FD Antenna Construction and Performance

[Josh]

On T2FD "directionality", I suspect the local ground plays a part. Some users see a diff between horizontal and slanted or vertical, while others report no diff. I also suspect the supporting structure as well as other antennae or conductors in the nearfield also play a role.

On the termination position with reference to the feedpoint, I suppose this is where the T2FD acts like a rhombic in that energy coming from the feedpoint is absorbed by the load, energy from elsewhere along the antenna not being absorbed as much, wich may also explain a lot about why the T2FD is legendary for poor tx performance. With a rhombic, you aim the termination and feedpoint inline with the target reception area - termination towards target, the antenna then rejects signals coming from the back.
Who knows, fun to consider and experiment with anyway.

[ChrisSmolinski]

Some additional data points:

The signal level is generally a bit on the low side. This is not an issue with the AFE822x which has an adjustable gain pre-amp, it's a little more of an issue with say the KiwiSDR which does not, especially with very weak signals.

I briefly played around with changing the tilt angle, making it closer to around 45 degrees. I did not notice any obvious change in signal strengths when looking at the MW band. I recorded a 500 kHz chunk at the upper and, and made the change about half way through (which was quick and easy to do - just untie the support rope at ground level and move it to another location). A quick check at HF, CRFX 6070, did not show any appreciable change in signal levels either. According to Cebik's analysis tilting the antenna does change the pattern and make it less symmetrical, but does not seem to improve the low angle radiation in any direction.

So I suspect a vertical orientation is still ideal, but tilt as needed. The question is where is the tradeoff? Your tree(s) are a fixed height. If you want to make the T2FD longer (for better low frequency performance), you need to tilt it more. When does making it longer but more horizontal start to hurt you? OTOH if your receiver has sufficient sensitivity, you do not need more signal, so you don't need to make it longer. 

[ChrisSmolinski]

On T2FD "directionality", I suspect the local ground plays a part. Some users see a diff between horizontal and slanted or vertical, while others report no diff. I also suspect the supporting structure as well as other antennae or conductors in the nearfield also play a role.

Yes, I suspect there's a lot of known unknowns as well as unknown unknowns affecting antenna performance. One reason you just have to "build and see how it works". 

[Exo]

Generally speaking, the radiation pattern for a T2FD antenna with a balun is the same shape as a dipole of the same length.
The only difference is in the efficiency gain or amplitude of the curve.

On HF, it is advantageous to get the feedpoint as high as possible, especially to get the radiation center above ground clutter and above receive RFI sources.
For a single support, the Inverted-V configuration gets the feedpoint at the top.

And it turns out that the TFD tends to do very well as an Inverted-V.
Structurally, the inverted-v configuration helps to manage the weight and wind loading of the coaxial cable, balun, spreaders, and termination resistor.
The inverted-v TFD is the most commonly seen HF antenna throughout most of South America and Southeast Asia, as radio ops who travel there have probably noticed

[Sealord]

Just for my understanding, I take it this is the direction the antenna 'looks' with the tilt?

               Top
                /
North <-  /  South
             /
       Bottom

[ChrisSmolinski]

The bottom of the antenna is about 15 ft to the north of the top. Whether this is giving me any real directionality is another story 

[Sealord]

Gotcha.

I still have my RF Systems T2FD...I never thought to hang it vertically, but could do so at a decent height with the pine trees I have (at least the full length of the 45'antenna if not higher ~70' possibly).  Hmm...might have to try it again to see if it will minimize the some of the local RFI I've been fighting with my vertical setup.

[i_hear_you]

Since reading this thread I've been giving the T2FD some serious consideration and research.  Of particular interest is the function of the resistor at the "far end" of the antenna.  Understanding its purpose should help in designing my own T2FD.

I found a very old post from this very forum describing (at least a portion) of the resistor's function which suggests why so many report the TF2D as having lower noise and better S/N than monoband dipoles:

The balancing resistor keeps the elements in perfect balance all the time in relation to the feedpoint, and THAT does a very effective job of canceling out local noise.

Having read elsewhere that "dummy loads have extremely flat SWR curves" and noting the T2FD is very wide-banded, the concern is how much energy is lost and how to minimize it.  The same post from above includes this:

In addition, at MOST, the balancing resistor might cause a 3 dB drop in signal (1/2 an S-unit).  Most of the time, it absorbs little, if any signal.  Almost all signal is fed to the feedpoint through the folded dipoles because of the slightly lower resistance at the feedpoint that at the balance point.  That is why for a 500 ohm feedpoint, you use a 540 to 600 ohm resistor in a terminated folded dipole.

As noted elsewhere in this thread, good performance from a T2FD depends heavily on the resistor value.  The above post gives an explanation for why that is.

Is finding the feedpoint impedance simply a matter of deploying a T2FD, running the shortest possible 50-ohm coax to an analyzer and noting the R in an R,X scan?

[Josh]

The resistor closes the loop with resistance, this is what makes the T2FD broadbanded aka aperiodic. If the resistor was replaced with wire, creating a folded dipole aka loop, the antennae would have swr peaks and nulls akin to other dipoles, but rejecting energy at twice the tuned frequency - a main characteristic of a folded dipole.

here are some vids on folded dipole experiments;
https://www.youtube.com/watch?v=WwJnXTVUrgI
https://www.youtube.com/watch?v=dsyuotZyU_g
https://www.youtube.com/watch?v=HmaLvSJKCOc

[WWBR]

I had something *vaguely* like this in the 80's and it was quite a nice antenna. I had RG8 on it instead of RG6 and no balun, but it was very similar to this antenna. I think that one had a 600 ohm resistor at the feedpoint. It was *almost* as good as my >400' random-chaos longwire but it was a lot quieter. Granted, QRM was nothing like we deal with now. But my mom had a pair of touch-lamps which plagued the longwire, and the terminated dipole was practically immune to it.

I may just try one of these!

Edit: The antenna I used was built from a long chunk of  2-conductor wire that was laying on the ground beneath power lines. I always assumed by it's small gauge that it was some kind of phone line. It was like two separated 24 g wires but very hard to bend. I had probably 75' of it. Had no plans for building it whatsoever, it was just an experiment after reading about folded dipoles in Pop Comm. Turns out Chris' post brought that old antenna out of my memory. It worked quite well so I never messed with it. 

[Traveling Wave]

Another interesting youtube video on the T2FD ......https://www.youtube.com/watch?v=Y473hOmif-E

[Pigmeat]

I had something *vaguely* like this in the 80's and it was quite a nice antenna. I had RG8 on it instead of RG6 and no balun, but it was very similar to this antenna. I think that one had a 600 ohm resistor at the feedpoint. It was *almost* as good as my >400' random-chaos longwire but it was a lot quieter. Granted, QRM was nothing like we deal with now. But my mom had a pair of touch-lamps which plagued the longwire, and the terminated dipole was practically immune to it.

I may just try one of these!

Edit: The antenna I used was built from a long chunk of  2-conductor wire that was laying on the ground beneath power lines. I always assumed by it's small gauge that it was some kind of phone line. It was like two separated 24 g wires but very hard to bend. I had probably 75' of it. Had no plans for building it whatsoever, it was just an experiment after reading about folded dipoles in Pop Comm. Turns out Chris' post brought that old antenna out of my memory. It worked quite well so I never messed with it.

Yep, two conductor phone line. You could have made fish hooks and pierced ears with that stress hardened copper.

A friend gave me three partial reels of the stuff in the 90's. I've still got most of two. The size makes it great for stealth antennas in HOA afflicted areas.

If you can find a multi-conductor reel there is usually a light blue coated wire. You can't see an antenna made of that stuff day or night unless you know exactly where you're looking.

[WWBR]

If you can find a multi-conductor reel there is usually a light blue coated wire. You can't see an antenna made of that stuff day or night unless you know exactly where you're looking.

There is a bundle of old phone line wires (bundle of about 25 wires) that run all through this house. Just the reg copper, but they must have put it in for alarm sensors or something, because they run into every room and the separate garage. I'll probably try to pull them all out when we redo the walls but it'll probably be some time before I can afford that project.

At my old place, the chaos longwire was sky blue wire and it was practically invisible from right beneath it.

[Josh]

I shudder involuntarily when someone mentions touch lamps.
So glad those things are out of style.

[ChrisSmolinski]

I did an experiment today. Well, two experiments.  First yesterday I moved the T2FD to a new location, about twice as far from the house, to hopefully reduce any RFI pickup. The new location also has more open space, so I can play with the antenna orientation. The top of the antenna is about the same height as before.

The experiment today: I set up the AFE822x SDR recording the upper end of the MW band. This gives me a number of relatively stable signals to monitor. I let it record for about 15 minutes, then I moved the bottom of the antenna, so instead of being nearly vertical, it is at about a 35 degree angle from vertical, with the bottom aimed roughly to the north, perhaps slightly east of north. Then let it record another 15 or so minutes.

I plotted out the carrier signal strength (2 kHz wide bandwidth) for several MW stations from the recording. Here are those plots, vertical axis is signal in dBm, horizontal is time, sorry no ticks, but each graph is the same length, and again the change is about midway:

Baseline noise 1715 kHz:


My semi local 1620 pirate WEDG which is to my north:


1470 WTTR which is about 10 miles to my south:


1480 WEEO 39 miles to my northwest:


1490 WARK which is 40 miles to my west:


1500 WFED 46 miles to my south:


1510 WWSM 49 miles to my northeast:


1520 WTRI 43 miles to my west south west:


1530 WCTR 58 miles to my east south east:


1540 WACA 48 miles to my south:


1570 WNST 30 miles to my east south east:


1650 WHKT 204 miles to my south:


1660 WWRU 172 miles to my northeast:


1680 WTTM 105 miles to my east:


1690 WPTX 101 miles to my south-southeast:


1700 WRCR about 180? miles to my northeast: