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Any experience with the TFD antenna?

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Post by Andrew Tue Sep 14, 2021 10:54 pm


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Post by Andrew Sun Sep 12, 2021 3:51 pm

Andrew wrote:Forgot, if you can/want, pick a copy of 4NEC2 and put together some antenna models using their effective size/layout/height, that will allow you to evaluate various setups and decide where to start or check the effects of whatever modification, ideally you will seek for relatively low radiation angles Smile

Oh and Harry, here's some food for thought

https://www.hamuniverse.com/kf9focfdipole.html

https://www.m0mcx.co.uk/how-to-use-dual-coax-feeders-as-ladder-line/

now, consider that a twin coax setup like that will result in about 100 Ohm CHARACTERISTIC impedance, but with the implicit transformations (and losses) and so on

So, it may be possible to use a (relatively) short run of twin coax between the antenna feedpoint and some point near ground, where one may place a balun and a good choke and go on with single coax feedline

I believe that the idea may be worth exploring, in this case the antenna may be a linear loaded (cobra) one, configured as a doublet, fed using a run of twin coax going to the balun/choke and then to the single coax feeder

I believe that the idea may be worth exploring

Add it a "loop on ground" (LoG) for RX only and you may have a perfect combo; as for the LoG my suggestion is to lay it down before it snows, it WILL work even if covered by some meters of snow, just in case, check this

https://tapr.org/loop-on-ground-log-antenna/

and look at the pic Smile

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Post by Andrew Wed Sep 08, 2021 7:29 pm

Forgot, if you can/want, pick a copy of 4NEC2 and put together some antenna models using their effective size/layout/height, that will allow you to evaluate various setups and decide where to start or check the effects of whatever modification, ideally you will seek for relatively low radiation angles Smile

[edit]

Had some time and wrote a quick "linear loaded doublet" model for the 4NEC2 program, the model is here

Code:

CM .
CM linear loaded doublet
CM .
CE

' symbols
SY freq=3.500                           ' test frequency
SY wave=(300/freq)                      ' wavelength
SY wqtr=(wave/4)                        ' quarter wave
SY redu=10                              ' loading reduction percentage
SY arms=wqtr-((wqtr*redu)/100)          ' total arms wire length
SY fspc=0.10                            ' feedpoint spacing
SY base=0                               ' height adjustment
SY wire=0.00125                         ' wires radius
SY vspc=0.05                            ' wires vertical spacing
SY leng=(arms/3)-fspc                   ' arms length (3 wires)
SY hgt1=9+base                          ' feedpoint height
SY hgt2=hgt1-vspc                       ' second arm height
SY hgt3=hgt2-vspc                       ' third arm height
SY segl=11                              ' segments for long wires
SY segs=3                               ' segments for short wires

' feedpoint spacer wires
GW  1 segs   0     0    hgt1      0  fspc  hgt1 wire
GW  2 segs   0     0    hgt1      0 -fspc  hgt1 wire

' arms wires
GW 10 segl   0  fspc    hgt1      0  leng  hgt1 wire
GW 11 segl   0 -fspc    hgt1      0 -leng  hgt1 wire
GW 12 segl   0  fspc    hgt2      0  leng  hgt2 wire
GW 13 segl   0 -fspc    hgt2      0 -leng  hgt2 wire
GW 14 segl   0  fspc    hgt3      0  leng  hgt3 wire
GW 15 segl   0 -fspc    hgt3      0 -leng  hgt3 wire

' arms junctions
GW 20 segs   0  leng    hgt1      0  leng  hgt2 wire
GW 21 segs   0 -leng    hgt1      0 -leng  hgt2 wire
GW 22 segs   0  fspc    hgt2      0  fspc  hgt3 wire
GW 23 segs   0 -fspc    hgt2      0 -fspc  hgt3 wire

' ground parameters
GE  -1
GN  2  0  0  0  13  0.005

' wires loading (copper)
LD  5   0  0  0  58000000

' feedpoint placement
EK
EX  0 1 1 0 1 0 0

' initial test frequency
FR  0  1  0  0  freq  0

' end of model
EN

just copy and paste the above code to a text editor and save it with a ".nec" extension, then load the model into the 4NEC2 program and you'll be able to play with the antenna model and parameters, the above model should be pretty self-explanatory, instead of starting from some pre-baked model, I calculate 1/4 lambda and then apply a reduction factor of 10% due to the linear loading, the resulting value is then divided by 3 and the spacing for the feedpoint and the center junction is subtracted, the antenna is placed at 9m from ground, but it's easy to change that; to see the antenna layout change the vspc value to (say) 1 (one meter) and reload the model, but to simulate it, keep the spacing at its original value

[edit #2]

Just in case, I ran a simulation of the above antenna model on some ham bands, and the resulting radiation pattern is shown here

Any experience with the TFD antenna? Lld

as you can see, the pattern on the 80m is mainly NVIS due to the low height of the antenna from ground, but it improves when going up in frequency, and even on 80m the 50° (from vertical) -3dB radiation angle could offer some pretty good long range contacts, the estimated gain ranges from 3.5dBi on 80m to 9.9dBi on 10m and the most interesting pattern is the one on the 20m band


Last edited by Andrew on Tue Sep 14, 2021 10:50 am; edited 2 times in total (Reason for editing : Added NEC model)

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Post by Andrew Wed Sep 08, 2021 5:12 pm

Hi there Harry, on holiday at the moment Smile so I'll keep this short

My suggestion is to try the "cobra" antenna built using 3 conductors cable and fed using a run of open line (ladder-450 or window-300) from the feedpoint to (near) ground, then use a balun+choke there to connect to the coax, the resulting doublet may surprise you

refer to this

https://www.nonstopsystems.com/radio/frank_radio_antenna_cobra.htm

for an overall idea, and also see

https://www.hamuniverse.com/cobraantenna.html

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Post by admin Wed Sep 08, 2021 9:20 am

Hi Ivan,

I once saw something similar about 30 years ago. It gave a VSWR of almost 1:1 on every HF amateur band. Interestingly it also gave a 1:1 VSWR continuously from 1MHz to 28MHz (and started to rise at 29.7MHz). In those days they did not hide much: the base was a metal box with a removable lid. Removing the cover revealed ferrite rings and a large power-resistor with fins on it.

As you pointed out, the VSWR was useless but all the reflected power was burned up in a dummy load, and the actual performance on the air was rubbish. If you have an antenna with, say, a 13dB loss then the antenna is still usable and you can have some good contacts with it, even if your 100 Watts in has 95% wasted and only 5 Watts used. Only a 2 S-point difference!!

I used to attend a radio club in England and when members compared antennas VSWR was nearly always the most considered criterion. Efficiency seemed to be taken for granted.

But you are 100% right.

Best regards from Harry - SM0VPO

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Post by Ivan Wed Sep 08, 2021 8:49 am

Admin wrote:... It is interesting that there are always loads of comments when one mentions antennas. Everyone has their own favourite ...
Hi Harry,
everyone has his antenna(s) in a different environment, so the same antenna works in a different manner for him.

What do you and all others think about this miracle ? I wonder what is inside the cylinder. They pour epoxy resin into it, so it is almost impossible to open. I hope it is not a 50 ohm resistor  Any experience with the TFD antenna? 1f60b  The rest of the set is quite a crap, not worth 500$.

VBR from Ivan OK1SIP

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Post by admin Tue Sep 07, 2021 9:08 pm

Hi Andrew,
I looked at the links and and it is a really interesting article. Then I sort of got occupied. But the antenna looks quite good.

Last week I was at (google.maps) "GIC Coffee, Rosersberg", the little mound of dirt at the side of the access road. I perch on that dirt hill many lunch-breaks, and last Week I had a contact in Japan with my 100000mW radio and the telescopic. This time I shortened the ground radials, made them different lengths and used 4 of them. The VSWR is absolutely perfect and I can work the world.

This is the same performance I want from home. Nothing electrical in the area, no QRM, just like using VHF.

But this winter I will be doing a lot of experimentig. I have even bought a ladder (for renovating the house) and now I can access that damn tree Smile 

My multiband antenna was a good try, and I have always had good results with MB dipoles. But in Lunda I had a multi-band but only three bands, 3.5, 3.8 and 14. Full dipole, no coiling or folding. I was spoilled rotten with all the space I could dream of.

So this winter I will be looking at:
- On the ground antennas for RX only (but the MB dipole is not so good for TX)
- Travelling wave terminated dipole
- Windom
- Variable length dipole

 Think those three will take the whole winter. I alsohad an idea for tuning the varible length dipole, a handheld oscillator with VSWR-meter at the bottom of the mast. Then I can watch while I tune it. Probably end up using marker tape on the guy rope for each frequency/band.

Thank you for the reply. It is interesting that there are always loads of comments when one mentions antennas. Everyone has their own favourite Cool  But I think I am falling out wth the MB dipole after I tried to push the technique to the ridiculous. It works fine, but (unexpectedly) not so good as a single dipole. I expected a bit of degredation, but I see a lot more than I can accept.

Best regards from Harry - SM0VPO

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Post by Andrew Fri Aug 27, 2021 9:31 am

Oh, and... Harry, did you consider the "poor" end-fed "random" antenna ?

A run of 25.6 meters of wire and some counterpoise will allow you (with a tuner) to cover all bands from 80 to 10 meters (it will work poorly on 160 since it's less than 1/4 lambda there), building it is easy and cheap (the most costly bits are the toroids for the 9:1 UnUn and the choke) and the antenna is almost invisible (in particular if you pick some neutral color insulated wire)

I don't know if you are "allowed" to string the wire from (near) your home to a tree, but if so, then you may be surprised by that simple, poor antenna; I've helped (ok, by mail) a friend in Greece to put up a random (he has a number of limitations in his place) and while at first he thought the antenna would at least allow him to make local contacts, he was surprised when he was able to make contacts with Japan, Brazil and then some with just 100W (at max) on the random

For some further informations, have a look here

https://www.sm7uzy.se/art/the_end_fed_long_wire_project.php

I disagree about his 9:1 UnUn and Choke design, but the remainder of the page carries pretty correct informations, so it may be worth a read; as for the choke, I already described it and that's ok for the random too, as for the UnUn, follow this design http://vk6ysf.com/unun_9-1_v2.htm but use an FT240-43 core and wind 9 trifilar turns, that will work over the whole HF range; by the way on some bands you may need to use the ATU, but that won't be a big issue, I believe

Keep the antenna feedpoint as high as possible, and as for the counterpoise, you may run a wire all the way to ground and then add some shorter radials (those don't need to be spread around like the ones of a vertical, just lay them down as possible)


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Post by Andrew Thu Aug 26, 2021 2:23 pm

Also see the TxFD section down this page

http://hflink.com/antenna/

HTH !

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Post by Andrew Thu Aug 26, 2021 9:46 am

Hi there, Harry, I'm very late on this, but I'm happy to hear you're getting better !!

Getting back on topic, if you want to go for a T2FD design, keep in mind that such an antenna was designed for "mil" use, where having frequency agility (w/o the need to adjust an ATU) was more important than wasting some power in the loading resistor (the "mils" could just feed more power to the antenna Very Happy) also, the antenna is mainly NVIS on lower frequencies, this doesn't mean that one won't be able to make DX contacts with it, but the main radiation angle of a "terminated folded dipole" won't help there

With the above being said, if you're going to put together a TxFD my suggestion is to consider a so-called T3FD (that's a T2FD using three wires instead of two) since such an antenna will be a bit more efficient and may be built shorter, a good guide to put together a T3FD is this paper

http://hflink.com/antenna/Build_a_T3FD_Antenna_by_Tony_Rycko_KA2UFO.pdf

which describes the whole design and build in great detail and which may be a good source for ideas and hints which may also be applied to a T2FD; notice that the author used the antenna with 150W w/o problems and also notice that the loading resistor (pack) is rated for 80W this should tell you something about the amount of power dissipated by the resistive load

Now, since I don't think you already built your TxFD, let me propose you another antenna which will still be multiband but doesn't use loading resistors, is short enough to fit your available space and is "quieter" than a standard dipole (at least in my experience); the antenna I'm referring to is the LLD (Linear Loaded Dipole/Doublet) which in USA is also known as "cobra" due to the "snaking" of the wires, the original LLD design from W4JOH is described here

https://www.hamuniverse.com/cobraantenna.html

and the above link is interesting since it carries some informations about the original design, in your case, due to space constraints, my suggestion is to go for this other LLD design

http://www.m0pzt.com/80m-cobra-doublet/

which, with an overall length of 12 meters, allows covering all the bands from 80 to 10 meters, the only things I'd recommend changing in the design are the spacers and the centerpiece, since the ones used by M0PZT aren't though enough to allow the antenna to stay up in icy winters (and that's important in your case  Very Happy), other than that, to feed the antenna I recommend using at least a run of 300 Ohm open line from the feedpoint to (almost) ground, there you will then put a 1:1 guanella choke/balun connected to the coax feeder, in some cases a 4:1 BalUn may be needed to get a better match, in such cases, the 4:1 will be connected to the 300 Ohm line and the output of the 4:1 will go to a 1:1 guanella and then to the coax, this is needed to tame the CMC which will inevitably flow on the coax braid (using chokes and grounding the coax before entering the building is common good practice in any case, indipendently from the antenna)

By the way, to use the LLD you will need an ATU on some bands, but I don't think it's a big issue, just a matter of finding and recording the ATU settings for each band and tuning the antenna will be a matter of seconds, on the other hand, the LLD will have better efficiency than the TxFD ... then ok, the TxFD could allow you to avoid the antenna matching unit; bottom line there's no free lunch, it's always a matter of deciding what's more important in a given setup, there's no "magic" antenna offering full coverage from 1 to 30MHz with good match (aka no ATU) and high gain Very Happy or, if such an antenna exists, it's probably built using wires of unobtainium and should be installed at night after some sacrifices and rituals  Laughing

And now, before I conclude, another possible idea which will fit your available space could be using two antennas, one for TX/RX and a second one for RX only, such a combo may be represented by a multiband vertical antenna (TX/RX) and a LoG (Loop on Ground - RX only), by the way the TX antenna may be whatever floats your boat, but in my experience, the combo with a separate, dedicated RX antenna works very well, in particular if one lives in an high RFI area

Well, that's enough I think, hope to have given you some further food for thought

[edit]


Had a bit of time and decided to quickly put together a simple 4NEC2 model for the cobra, the image shows the radiation pattern for a horizontal LLD at 10 meters height

Any experience with the TFD antenna? Lld

(please open https://postimg.cc/2VDjHJSn for the full size image)

as you can see, the pattern on 80 is almost vertical and lowers when going up in frequency


[edit #2]

As for the BalUn and Choke (the latter may also be used as a 1:1 BalUn btw), here are my suggestions

* Balun (4:1)

Refer to this schematic http://vk6ysf.com/balun_4-1.htm but use an FT240-61 core, pick two teflon insulated wires and twist them together, use the resulting twisted pair to wind 15 turns evenly spread over the toroidal core, then connect the wires ends as shown in the VK6YSF schematic, the resulting BalUn will easily support 400W (very conservatively - effectively much more) and will serve you well over the whole HF range

* Guanella choke/balun (1:1)

Refer to this schematic http://vk6ysf.com/balun_guanella_current_1-1.htm but use an FT240-43 core, pick a length of RG-142 coax and use it to wind a total of 18 turns, that is 8+bypass+9, such a choke/balun will again serve you well over the whole HF range and will offer very good choking against CMC, it would be a good idea using at least two of those chokes, one right after the BalUn (if used, whatever is used), at the antenna, and a second one right before the coax enters the building; if desired, more chokes may be added along the coax feedline (they won't hurt)

As a note, if using the 4:1 BalUn, both the BalUn and the choke may be hosted inside a single waterproof box, then additional chokes may be hosted in separate boxes with coax connectors to add them wherever desired



Last edited by Andrew on Thu Aug 26, 2021 11:08 am; edited 7 times in total (Reason for editing : added balun/choke infos)

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Post by admin Thu Aug 19, 2021 8:55 am

Hi Glenn,
As I stated earlier, these links were very good and gave me a lot of food-for-thought, and some practical information.
There is a lot of information available, especially the article published in RadCom by G8JNJ.

Of all the terminated antenna variants, each has it's own benefits and disadvantages. Every radio ham has a different physical location properties which needs some careful thought in the construction of an antenna. I think I now have sufficient information to create an antenna suited to my home location, using the terminated antenna principle. In my situation I have to use horizontal and high. Vertical is not an option for me at home.

Thank you for sharing the information and links.

Best regards Harry - SM0VPO

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Post by Glenndk Wed Aug 18, 2021 3:18 pm

Admin wrote:Hello Glenn,
Thank you very much for the response. Yes I am on the road to recovery. Funny how things start happening to the body at 70 years!! But it is all a part of ageing.
...
I am mainly interested in frequencies above 14 MHz, so efficiency at the lower frequencies is not so much of a problem for me. I have always used resonant, single-band antennas, where the efficiency is much greater. At resonance the same signal passes back and forth along the wire many times, radiating just a small amount with every pass (2% to 5%, depending on Q and accurate pruning). However, I have found that a multi-element dipole has a reduced efficiency, depending on the number of elements for other frequencies that are added.

From what I understand now, if the total length of wire in a T2FD antenna is greater than one wavelength, then efficiency is around the 15% to 20% region. If the antenna is longer than 1 wavelength the the efficiency can rise to 40% - 50%. The single-band dipole efficiency can be over 80% but my multi-band dipole seems to have an efficiency of closer to 20% compared to a single-band dipole on the same mast (experiments done at 14MHz). In other words, the TFD and a multi-band dipole are about the same, but the TFD becomes more efficient at higher frequencies.
...

Hi Harry

Please read this (from https://sm0vpo.forumotion.com/t228-indoor-tc2m-g8jnj-broadband-shortwave-antenna-tested#1175 ) - TC2M ought to be approximately 100% higher efficiency for same width as T2FD at 40-10 meter bands:

Look at these efficiency curves for the T2FD:
http://www.korpi.biz/T2FD_NVIS.pdf
backup:
https://web.archive.org/web/20200109052409/www.korpi.biz/T2FD_NVIS.pdf
Quote: "...
We tested it over average ground type; only minor changes were seen on simulations and SWR measurements.
..."

411 TERMINATED FOLDED DIPOLE ANTENNA:
https://www.codanradio.com/wp-content/uploads/Codan-411-Terminated-Folded-Dipole-Antenna_Screen.pdf
backup:
https://web.archive.org/web/20210818131946/https://codancomms.com/products/411-terminated-folded-dipole

http://www.eham.net/ehamforum/smf/index.php?topic=68475.0
Quote: "...
That is a short Wideband Terminated Dipole. Yes I have built, modeled, and written about this type of antenna. The Buxcomm specified gain is at 28 MHz. These antennas are simple to model with NEC-2:
T2FD radiation efficiency:
80 meters: 4%
40 meters: 16%
20 meters: 25%
15 meters: 35%
10 meters: 55%
..."

.

Please compare with:
http://web.archive.org/web/20150529061427/http://www.tc2m.info/TC2M%20HF%20Vertical%20G8JNJ.pdf

(actually it is problematic to campare the efficiency of - a vertical with GP - with a horisontal antenna near ground...)

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Post by admin Wed Aug 18, 2021 11:43 am

Hello Glenn,
Thank you very much for the response. Yes I am on the road to recovery. Funny how things start happening to the body at 70 years!! But it is all a part of ageing.

It was really interesting to read your previous post and follow all your links. This time I read with experience, and I do regard you as having experience with this type of antenna.

I know that many people have had a negative response to TFD antennas, myself included, and I think it is probably more religious than scientific, or something like that. It was the thought of a resistor burning up hard-earned RF. I also noted that one guy had his submitted technical articles refused, simply because these antennas do not need and ATU, so magazines with adverts for ATUs are biased against them.

I am mainly interested in frequencies above 14 MHz, so efficiency at the lower frequencies is not so much of a problem for me. I have always used resonant, single-band antennas, where the efficiency is much greater. At resonance the same signal passes back and forth along the wire many times, radiating just a small amount with every pass (2% to 5%, depending on Q and accurate pruning). However, I have found that a multi-element dipole has a reduced efficiency, depending on the number of elements for other frequencies that are added.

From what I understand now, if the total length of wire in a T2FD antenna is greater than one wavelength, then efficiency is around the 15% to 20% region. If the antenna is longer than 1 wavelength the the efficiency can rise to 40% - 50%. The single-band dipole efficiency can be over 80% but my multi-band dipole seems to have an efficiency of closer to 20% compared to a single-band dipole on the same mast (experiments done at 14MHz). In other words, the TFD and a multi-band dipole are about the same, but the TFD becomes more efficient at higher frequencies.

All of this puts it into perspective. I only have space for two horizontal dipoles, if I physically mount them at 90° to each other. So my choices for my small garden are probably:

1 - Try a TFD antenna and get something working for all bands, accepting the reduced efficiency at low frequencies.
2 - Choose perhaps only two bands and have one tuned dipole for each of the bands I am interest in.
3 - Choose only one "serious" band with a tuned dipole, and use a TFD for other bands.

Another point is that an antenna that is 25% efficient will only give one S-point reduction. Most of my contacts this year have been RS 5/5-ish, so I can afford to loose only one S-point. If the TFD also reduced noise then it will possibly open up the 7MHz band for me.

__________________________


Then from your information I saw the Mono-pole cage dipole - TC2M. That sparked my interest a lot more. You wrote "The TC2M return wire is NOT visible in the far field (is in a Faraday wire cage). So the return current will NOT subtract from the current in the wire near the feed point. This makes the TC2M more efficient.". I will have to do a lot of thinking about that. My garden is very small so the vertical mono-pole could be a good solution.

I did some experiments over the past 8 months and I found that from home I am overshadowed by a huge tree, surrounded by bushes and beside a communal garage. From the experiments I did I found that a vertical 1/4-wave was nowhere near as good as a horizontal dipole at 7m. That is why I built the horizontal multi-band. It enabled me to become QRV on all bands and have a low visual impact.

When I operate /P I use a vertical 1/4-wave (with ground radials) sitting on the ground, and that gives me much better results then the multi-band dipole at home. A month ago I found a lovely mound of earth in an open field, about 4 meters high, where the local builders have simply piled up unwanted dirt. Putting the 1/4-wave on that gave me even better results.

So I wonder if a horizontal, cage, terminated dipole will give any better results? That can probably be erected in place of the multi-band dipole so the kommun and neighbours will not see any real difference from that they have already accepted.

It looks as though I shall have some exciting winter antenna projects to do Very Happy

Thank you for all your comments and sharing your knowledge and experience. I really appreciate this. Now I have more to do. I apologize for long-winded reply, but you know me; sometimes difficult not to be a bit verbose Wink 

Have a nice day, and very best regards from harry - SM0VPO

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Any experience with the TFD antenna? Empty Re: Any experience with the TFD antenna?

Post by Glenndk Tue Aug 17, 2021 4:39 pm

Hi Harry

I just read that you are better now. I hope you are over most of your ordeal.

Admin wrote:
Hi guys,
Has anyone any experience with the Terminated Folded Dipole antenna (TFD)?

Yes and no :-)

I have many years experience with the TC2M/G8JNJ broadband shortwave antenna. TC2M/G8JNJ antenna is a sort of improved T2FD:

I did even wrote about the TC2M/G8JNJ antenna here :-) :
https://sm0vpo.forumotion.com/t228-indoor-tc2m-g8jnj-broadband-shortwave-antenna-tested

...
Admin wrote:So my questions for discussion are:

1 - Has anyone ever done any work with the TFD or the T²FD?

Yes and no :-)

Admin wrote:2 - Does it REALLY reject QRM like the OTG antenna?

Sort of - because T2FD and TC2M are (almost) closed wire loops and they both are low loaded Q antennas, they do not pick up as much QRM as an unterminated wire antenna. (near field noise)

An unbalanced vertical or a horisontal balanced TC2M is more efficient (especially at 160-60 meter) than a T2FD of equal length.

The reason is that the T2FD return wire is visible in the far field. So the return current will subtract from the current in the wire near the feed point.

The TC2M return wire is NOT visible in the far field (is in a Faraday wire cage). So the return current will NOT subtract from the current in the wire near the feed point. This makes the TC2M more efficient.

(The unbalanced TC2M vertical is much more efficient with a suitable groundplane, than a horisontal balanced TC2M.)

Admin wrote:3 - Does open-wire feeder have a tendency to pick up more QRM?

The theory says so. Practice seems to back it up.

Admin wrote:4 - Is this another antenna that requires "oofle-dust and magic"?

Everything that is not scientific explainable (yet) is magic. :-)

Every antenna with unbelievable high gain is magic - until disproved. :-)

...

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Glenn / OZ1HFT

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Any experience with the TFD antenna? Empty Any experience with the TFD antenna?

Post by admin Tue Aug 17, 2021 1:47 pm

Hi guys,
Has anyone any experience with the Terminated Folded Dipole antenna (TFD)?

I have seen several articles on the WWW, and more recently in this month's QTC, and I read that the antenna can basically be any length - the longer it is so the more efficient it can become. With a resistor at the other side burning up RF power, there is no reflected power so VSWR is good at all frequencies, and some reports say it even has a high immunity to QRM.

For me it looks as though it could be one of the winter projects, since the multi-band dipole I have is really a compromise. Single-band dipoles are always the best IMHO.

What I am concerned about is that some of the TX power is burned at the resistive termination. But a loss of 3dB is acceptable for a wide-band antenna, and I am sure that my multi-band dipole has greater losses. I would expect that the TFD antenna losses would be the inverse of frequency, since the wire elements become shorter relative to the wavelength as the frequency reduces.

I have seen gain figures quoting +5dB at 30MHz, 0dB at 14Mhz and -5db at 7MHz for a horisontal "Inverted-V" configuration that is 45' (15m) from end-to-end, and a lower usable frequency of 1.8MHz!! santa

I think it could be interesting to try this antenna in my small garden, mainly because of the QRM claims, and no need to tune. My 6-band dipole was a real monster to tune, but when I got it right it was ok.

I am sure I can also get a bit more gain by reducing the feeder losses. I presently have 50m of RG-58 ¼" coax, so perhaps it is time to think about investing in some ½" (12mm) cable, or checking how open wire feeder works.

So my questions for discussion are:

1 - Has anyone ever done any work with the TFD or the T²FD?
2 - Does it REALLY reject QRM like the OTG antenna?
3 - Does open-wire feeder have a tendency to pick up more QRM?
4 - Is this another antenna that requires "oofle-dust and magic"?

Very best regards from Harry - SM0VPO

PS - Oofle dust was a potent magical compound used in the 50's and 60's by a guy called "Sooty"
Any experience with the TFD antenna? 9k=

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