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Author Topic: New T2FD Antenna Construction and Performance  (Read 27995 times)

Offline ChrisSmolinski

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New T2FD Antenna Construction and Performance
« on: July 26, 2019, 1913 UTC »
I previously had a 132 ft long T2FD antenna circa 2008-2015 or so, that was eventually destroyed in a storm. It was a reasonably good performing antenna, and I've wanted to put another back up, which I just did this morning.

The new antenna is shorter, it is about 58 ft long, I made it with the wire from an 80 mb dipole that I was never really happy with, because it was fairly low, and the location wasn't great for getting it higher due to nearby trees.  It's made with #16 insulated hookup wire, my antenna wire of choice lately. You can get 500 ft spools on eBay for about $30 shipped if you watch for deals.

The spacers are 20 inch long pieces of 1" PVC pipe, because I happened to have some of that length as leftovers from another project. Each has two small holes drilled near each end for the wire to pass through.  The old T2FD used wooded spacers, which eventually broke.  There are 5 of them one in the center (which also supports the balun), one on each end, and one in the middle of each span.

The balun is a 4:1 [EDIT: I think it is really a 9:1], actually the one from the old T2FD. I'd normally use one of my Jellyfish transformers but I am out of them right now and didn't want to make a new one and wait for the resin to harden  ;D I think a slightly higher impedance ratio will be better, so it will be interesting to switch out the balun in a few days and see what the difference is. Coax feed is my usual 75 ohm RG-6 which goes from the balun down to a grounding block, underground in conduit, comes above ground with another grounding block, then up to the shack.

The terminating resistor is made from a pair of very old carbon composition resistors, 1 watt each, which are just over 900 ohms when used in parallel.

I did some research online as to the optimum tilt angle, as well as running some antenna models. It seems the best performance is from a nearly vertical orientation, not tilted at 45 degrees or so as many do. The old T2FD was actually not very far from nearly horizontal, as it was 132 ft long and I don't happen to have any redwoods in my yard.  ;D

I shot a line over a tree last night with the EZ Hang (it went over perfectly first time, how often does that happen?).  Based on the known length of the antenna and the measurements I was able to take at ground level, I could estimate that the top of the T2FD is about 59 ft high. The low end is 3 ft high and 15 ft horizontally from a vertical orientation. That works out to about a 15 degree tilt from vertical, oriented to the northeast.

Performance:

I'm surprised how well it works actually, given the size. It works even down to the MW band, I was able to hear 530 from Canada at 1700 UTC in the afternoon. And the 1620 pirate from about 10 miles away. Both with signal levels equal to the NE beverage antenna. And on the high end of HF, it's incredible on the 11m band. Much stronger signals than my other antennas.  On the 49m band it picks up CFRX 6070 and WBCQ 6160 (the only signals there now) about the same as my 43/48m folded dipole antenna, perhaps 6160 is even a hair better. Difficult to tell with 6070 as the signal is so strong anyway. It will be more telling shortly when the European DX starts to roll in on 49 meters, and eventually pirates on 48 and 43 meters.  I'll write a followup reply post with more performance information after using it for a while.
« Last Edit: July 27, 2019, 1800 UTC by ChrisSmolinski »
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 900 ft Horz skyloop / 500 ft NE beverage / 250 ft V Beam / 58 ft T2FD / 120 ft T2FD / 400 ft south beverage / 43m, 20m, 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline Josh

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Re: New T2FD Antenna Construction and Performance
« Reply #1 on: July 26, 2019, 1933 UTC »
Nice work, thanks for reporting it. I was gonna say to test for the optimal load via a variable resistor but if it's that far up in the air it'll be fun tweaking the pot every time you want to run a test. I think the normal load is 600 or 400, where the balun you used would seem more practical, maybe you stumbled onto the secret combination.
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Offline ChrisSmolinski

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Re: New T2FD Antenna Construction and Performance
« Reply #2 on: July 26, 2019, 1956 UTC »
SWR and impedance plots taken with the RigExpert here in the shack. I updated the first post to mention my use of 75 ohm RG6 coax.



Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 900 ft Horz skyloop / 500 ft NE beverage / 250 ft V Beam / 58 ft T2FD / 120 ft T2FD / 400 ft south beverage / 43m, 20m, 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline Josh

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Re: New T2FD Antenna Construction and Performance
« Reply #3 on: July 26, 2019, 2021 UTC »
Well I think you've found the sweet spot.
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Offline ChrisSmolinski

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Re: New T2FD Antenna Construction and Performance
« Reply #4 on: July 26, 2019, 2026 UTC »
Well I think you've found the sweet spot.

Yeah at this point I am hesitant to  change anything.
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 900 ft Horz skyloop / 500 ft NE beverage / 250 ft V Beam / 58 ft T2FD / 120 ft T2FD / 400 ft south beverage / 43m, 20m, 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline Josh

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Re: New T2FD Antenna Construction and Performance
« Reply #5 on: July 26, 2019, 2220 UTC »
Do you have any ferrets  on the feedline to decouple it from the antennae?
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Offline ChrisSmolinski

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Re: New T2FD Antenna Construction and Performance
« Reply #6 on: July 26, 2019, 2226 UTC »
Do you have any ferrets  on the feedline to decouple it from the antennae?

No, that's what the balun is for  ;D
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 900 ft Horz skyloop / 500 ft NE beverage / 250 ft V Beam / 58 ft T2FD / 120 ft T2FD / 400 ft south beverage / 43m, 20m, 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline Josh

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Re: New T2FD Antenna Construction and Performance
« Reply #7 on: July 26, 2019, 2230 UTC »
That's a pleasant thought.
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Offline pjxii

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Re: New T2FD Antenna Construction and Performance
« Reply #8 on: July 26, 2019, 2336 UTC »
Nice!  I'm a big fan of the T2FD for a receiving antenna.  I used the RF Systems version a number of years ago, very, very low noise even in a suburban location with a wide coverage range, as you pointed out.  I'd try building one myself but I still have two brand new RF System T2FDs that I bought on closeout so no point really.

Interesting about the angle you used (vertical). I found that reception was equally good either near horizontal or tilted close to 30 degrees. Maybe Countryman's original 30 degree design was in regards to transmitting for his intended purpose?

Offline i_hear_you

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Re: New T2FD Antenna Construction and Performance
« Reply #9 on: July 27, 2019, 0210 UTC »
Does your coax drop straight down from the feedpoint?

Offline ChrisSmolinski

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Re: New T2FD Antenna Construction and Performance
« Reply #10 on: July 27, 2019, 1235 UTC »
The coax comes off the balun at about a 45 degree angle. Speaking of the balun... I was puzzled about how good the impedance match / SWR was, especially since the impedance of the T2FD should be around 900 ohms. I thought about the history of that balun again, and now I think it is a 9:1 not 4:1. If I lower the T2FD for some reason I will measure the impedance ratio.

Cebik's analysis of the t2FD doesn't show much benefit for tilting it: http://the-eye.eu/public/Books/Electronic%20Archive/ModelingTheT2fd.pdf

Quote
Nothing in the models shows any advantage to tilting the antenna with respect to skip communications or reception. Perhaps the only advantage may be mechanical, for those lacking a suitable high support from which to hang the antenna vertically.

I like to think of the T2FD as having the low angle pattern benefits of a vertical antenna, without the usual warts that make verticals unsatisfactory in most cases (more noise, and difficult or nearly impossible to get a decent ground radial system without a huge effort).

What are the dimensions of the T2FDs you have?

I'm pretty happy so far with the performance. I am tempted to try tilting it to the south, to see if it really does change directionality. I can just use the other end of rope over the tree.

If I set up a second KiwiSDR, I think this antenna might go on it.
« Last Edit: July 27, 2019, 1346 UTC by ChrisSmolinski »
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 900 ft Horz skyloop / 500 ft NE beverage / 250 ft V Beam / 58 ft T2FD / 120 ft T2FD / 400 ft south beverage / 43m, 20m, 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline Exo

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Re: New T2FD Antenna Construction and Performance
« Reply #11 on: July 27, 2019, 1649 UTC »
  I found that reception was equally good either near horizontal or tilted close to 30 degrees. Maybe Countryman's original 30 degree design was in regards to transmitting for his intended purpose?

The original Countryman T2FD antenna was tilted to help offset the imbalance problems the design had due to its high impedance open wire feed.

In other words, they had a lot of transmission line radiation that showed up as nulls and peaks in the measured radiation pattern.
Tilting the flat top helped to fill in those nulls.

There was no balun in the original designs.
The open wire feedline was not decoupled from the antenna at all.

Ferrite baluns, which came along later, were applied in updated Terminated Folded Dipole designs.
Using a ferrite balun provides feedline isolation over a broad spectrum, eliminating feedline radiation in the TFD.

There is no magic tilt (and no need for tilt) in a TFD that has a balun.

 
« Last Edit: July 28, 2019, 1847 UTC by Exo »
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Offline Josh

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Re: New T2FD Antenna Construction and Performance
« Reply #12 on: July 28, 2019, 0041 UTC »
As I recall, USN NRL (Naval Radio Labs) devised the T2FD for shore station listening posts and the NSA picked up on the design as being much more portable than the typical rhombics then favored by signals intelligence orgs. One slang for the T2FD is squashed rhombic, perhaps the desirable features of rhombii were criterion for a new design that featured portability and ease of installation over the rhombii for field use. Much easier to install the lowly T2FD above a embassy or other govt building than a full size rhombic.

Collins Radio took a look at rhombics for some clandestine bcaster and decided to do away with the loss of the terminating resistor. They made a dual rhombic with a matching network in place of the load and doubled their output over rhombics by reflecting the rf back to the antenna rather than heating a load. Of course, that then eliminated the aperiodic nature of the rhombic.


From the swr graphs, with particular respect towards the lack of peaks and nulls, it seems Mr Smolinksi has perfected the art of the T2FD aperiodic antenna.
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Offline i_hear_you

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Re: New T2FD Antenna Construction and Performance
« Reply #13 on: July 28, 2019, 0100 UTC »
I can't stand a 60' T2FD on end at my QTH, but I could slant one that is significantly longer, so my question is: what do I lose from breaking from vertical, and what do I gain from adding length to the antenna? Does length add gain to lower frequencies?  Does less slant make it more omnidirectional?

Offline Josh

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Re: New T2FD Antenna Construction and Performance
« Reply #14 on: July 28, 2019, 0110 UTC »
Does length add gain to lower frequencies?

My B&W T2FD was 90ft long, covered 160m and up. Their shorter version was good from 80m. They seem to have a sharp knee in the swr that rises dramatically at the low end of the spectrum as a function of the design, this can be seen in Mr Smolinksi's charts.
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