Technical Topics > Equipment
T2FD Antenna
Tube Shortwave:
By the way... I just read your thread on QRZ. The two 'experts' that replied are showing their lack of knowledge, and a sharp prejudice for something they have not tried themselves.
If they had a shred of knowledge in this department, they would know that a balancing resistor on the OPPOSITE side of a folded dipole does not absorb "half the transmitter power" at all. If that were true, that resistor would blow up every time I run 1500 watts on CW. In fact, at most, the resistor will absorb 1/4 of the transmitter power if the antenna is that far out of resonance on my chosen operating frequency. The other 3/4 is radiated with a very small amount coming back (<10%). Most of the time, the balancing resistor isn't doing very much.
And, none of those jerkoffs even considered the receiving characteristics of the antenna (which is what it was actually designed for). 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. 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.
I could go on and on, but I have a job, and a life. Those 'expert' pinheads need one of each too.
See? Hams really jerk my chain!
73, TS
Seamus:
About the only thing I keep my (extra) license active for these days is to keep people from grousing about the legality of my data and video links. Enough hams can't cope with the idea that someone one third their age is allowed to play with "their" radio waves, or just might actually know something about RF that it almost isn't worth even listening around the bands anymore, let alone transmitting. If you can manage to find someone who doesn't go on about how you "can't be a REAL ham because...", then about all that's left for them to talk about is their medical problems. Beyond that, it's "call, signal report (always an inaccurate 5x9), weather, 73".
Just last week, one of the local FM repeaters was all abuzz with people offering their valuable opinions on which coax a new ham should use to hook up his 2-meter ground plane. One of the "experts" - I know he was an expert because he mentioned several times that he's been a ham for almost fifty years now - repeatedly told him that you didn't need to worry about coax line loss. It was unimportant, he said, because as an example, he's got 10 or 12 dB loss on his antenna, and "you can't hardly even hear a twelve dB sound if you're tryin'". Even so, he said that he just makes up for it by talking louder into the mic, and everyone can hear him just fine (his audio was clipping pretty badly, actually). The saddest part was, NOBODY contradicted him; several even agreed wholeheartedly.
Other "wisdom" I have heard discussed on the radio recently:
* RF radiation and ionizing radiation are the same thing (because they're both called "radiation", you know).
* There's no difference between the different types of coax - it's all just price.
* SWR doesn't matter with modern radios - they can pump full power into any kind of antenna (no mention of automatic power reduction when high SWR is encountered).
* You've GOT to have an SWR of 1.1 or less in all cases, or you'll burn up your finals within minutes.
* You can't use a computer and a radio together because they'll blow each other up.
* You can't build your own antennas or radio gear, because the people who make commercial units have access to "special wire", materials, and "design computers" to make them work.
* Regular horizontally-strung dipoles are non-directional (all cases, regardless of frequency or height above ground).
* It's impossible to operate HF from a moving vehicle, because the motion will cancel out the longer wavelengths.
* You can't polarize an FM signal (apparently, horizontal, vertical, RH and LH circular polarization are all just a bunch of hooey, despite demonstrable effect).
* The one biggest thing that can make or break a hamfest is having enough electric scooter carts available for rent (out of all of the above, this is the one item I believe to be true).
Listening to the local repeaters can be hilarious at times, but it doesn't take long before it just becomes too depressing to bear anymore; my dual-band mobiles are most often used for listening to the local EMT dispatchers these days.
W7LPN:
Thanks again. I am not an RF engineer. I know a few of them & they make more sense than any of these "Hambones" you speak of. When he was criticizing the effects of the non-terminating resistor, it sounded like puffed up BS. Sooner or later you can recognize the truth and the level heads in a discussion. Tahnks , & 73.
Tube Shortwave:
First of all, Seamus- You literally had me laughing out loud! I actually spit on my screen when I read the part about the electric scooters! Man, that was funny.
W7LPN- Yes, you are correct. It is pretty easy to spot the 'real' experts sometimes.
W7LPN:
I'm searching for the truth. Please read. :)
W8JI
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Join Date: Oct 2003
Posts: 2,853
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Any one additional comment you can pass back to "tube shortwave" about this:
"If they had a shred of knowledge in this department, they would know that a balancing resistor on the OPPOSITE side of a folded dipole does not absorb "half the transmitter power" at all. "
That wasn't the statement at all. He isn't even honest enough to quote someone accurately.
The statement I made (it is in black and white here on the Zed, so there is no debating he misquoted) is the resistor sometimes dissipates less than half of the power applied to the T2FD antenna, and on other bands it dissipates much more than half the applied power. Nothing worse than a critic who fabricates the very statements he is openly critical of.
He goes on to say:
"If that were true, that resistor would blow up every time I run 1500 watts on CW. In fact, at most, the resistor will absorb 1/4 of the transmitter power if the antenna is that far out of resonance on my chosen operating frequency. The other 3/4 is radiated with a very small amount coming back (<10%). Most of the time, the balancing resistor isn't doing very much."
That's clearly wrong, as any analysis or measurement has shown. What "tube shortwave" is unaware of is the owner of Amp Supply found that antenna design in some publication and decided to build it commercially. I believe he eventually cooked up a deal with B&W to buy the "design" of that, and a terminated V, antenna. The original matching transformers were the filament chokes from amplifiers, wound at Prime Instruments.
I actually directly measured the dissipation in the resistor and it was pretty high overall.
There is considerable mutual coupling between the dipole half with the resistor, and the dipole half with the feedline. Anyone who knows anything about antennas understands mutual coupling of two very long wires (90 feet or so long) spaced a few inches apart is extremely high, plus the ends of the wires are directly connected. The resistor is, for nearly all practical purposes, directly connected across the feedline.
The power division depends largely upon the unterminated impedance at the feedpoint. On bands where the antenna is a conventional half-wave or 3/4 wave dipole length, that impedance is low and the resistor has limited effect on the system. On bands where the feedpoint would have a high unterminated impedance, the resistor soaks up most of the energy.
This is why the SWR stabilizes. What would be a terrible SWR on some bands is reduced by diverting energy into the resistor on those bands.
When testing the antenna as a consultant doing work at Amp Supply, I built an antenna at home and ran the "resistor" side through ladder line to a dummy load. I had an RF current meter in series with the load, and measured load current. I still have the huge resistor I used here, I use it as a dummy load for testing high impedance balanced systems. The measured result was basically as LB Cebik and many others have analyzed and published.
If you search the web you will find many articles telling exactly what the T2FD antenna is, and what it is not.
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