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Author Topic: Difference Between a Terminated Folded Dipole, G5RV, and Cobra UltraLite  (Read 7212 times)
AK4SK
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Posts: 150




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« on: February 26, 2012, 07:21:34 PM »

What is the difference between a terminated folded dipole (TFD), G5RV, and Cobra UltraLite? I don't have the room for the full size (160m-10m) versions of each so I'm comparing the 80m-10 versions of each. Does the Cobra antenna have a more generic name? I looked at vertical vs. horizontal antennas and a multiband horizontal seems like the best option for my current needs and installation restraints.

What are the advantages and disadvantages of each of these? About all I understand at this point is that the TFD looses a lot of power in the resistor at some frequencies but that it requires no tuner from 80m-10m.
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N4JTE
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« Reply #1 on: February 26, 2012, 07:59:27 PM »

If you don't have room for a full size G5RV or the other ones, than describing in detail the differances would be best accomplished by reading about them in various antenna books.
Your situation may be better served by putting up the highest, longest, ladderline fed wire you can fit and use a tuner to obtain multiband use.
Bob
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N4JTE
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Posts: 1147




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« Reply #2 on: February 26, 2012, 08:06:57 PM »

Perhaps my article might be of some help, http://www.eham.net/articles/27400
Regards,
Bob
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WB6BYU
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Posts: 12980




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« Reply #3 on: February 26, 2012, 08:37:05 PM »

Here are some useful articles to start:

http://vk1od.net/antenna/G5RV/index.htm
http://www.w8ji.com/g5rv_facts.htm
http://www.cebik.com/content/a10/wire/g5rv.html
http://www.cebik.com/content/gup/gup6.html

http://vk1od.net/antenna/cobra/index.htm

http://www.cebik.com/content/a10/wire/t2fd.html

The articles at cebik.com are from the late W4RNL, and require free registration to access.
It is worth it, however, as there is a wealth of antenna information on his site.


You're right about the T2FD - it has high losses, especially once the antenna is shorter than
about 1/3 wavelength.  While it does radiate on 80m, it will be at QRP levels with a 100W rig.
On the higher bands you probably will still get 25 to 35 watts radiated.  They are popular with
military stations and others who need to cover a wide range of frequencies (rather than specific
bands like hams use) and who want an antenna that doesn't require much technical ability to
set up.  It's not one I would generally recommend for ham use.

The Cobra antenna is sort of a linear-loaded dipole, but the folded sections are a significant
portion of a wavelength long.  The fold-back arrangement is rather inefficient on some
frequencies, a shown in VK1OD's analysis.  This is because the currents in one wire will tend
to cancel the currents in the other wires.  This is worst at specific frequencies.

In both of these cases the antenna itself has high losses (at least at some frequencies.)  In
the case of the G5RV the antenna itself is efficient, but presents a high SWR on some bands
that can cause more power to be lost in the feedline.  In this case, the shorter and lower loss
the coax you use, the better the efficiency.  If you can put your tuner right at the bottom of the
twinlead section, or run twinlead all the way to a tuner in the shack, it can have good efficiency
on all HF bands.  For best results it should have a 1 : 1 current balun at the junction between
the twinlead and the coax, and you'll need to use a tuner on most bands.

A variant of the G5RV is the ZS6BKW, which is explained in the third section of W4RNL's
article.  It is somewhat shorter and works all HF bands except 80m, 30m and 15m (again,
due to SWR limitations when used with coax.  It will work better when the twinlead is fed
directly from tuner.)


Depending on your needs, there may be other antennas that work for you.  When trying to
fit in a small space, sometimes you need to custom-build an antenna for the space available
and the combination of bands that is most important to you.  In other cases a straight dipole
(or doublet) fed with twinlead will work - the length isn't critical as long as it is long enough
for the lowest operating frequency.  You may be able to build such an antenna with loading
coils for 80m that allow for a shorter antenna.  Such an antenna needs a tuner.
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AK4SK
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Posts: 150




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« Reply #4 on: February 27, 2012, 06:01:50 AM »

Wow, thanks for all of the resources. I will read through it all when I get the chance and let you all know if I have any more questions.
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WA9YSD
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Posts: 138




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« Reply #5 on: February 27, 2012, 08:51:08 AM »

I had RFI issues with the G5RV due to closeness of proximity. Long Story. It does work well though. Because of the RFI issues I switched to folded dipoles.

The others antennas you mentioned are inefficient not worth the trouble.

You may want to consider loading coils or traps. Not sure what will happen with combining traps and loading coils on the same antenna. Never tried that. Seams that a trap is not that much different than a loading coil except a trap has a tuned frequency. Add enough in series and the antenna is shorter. Each band section is tuned to the desired freq. 

Loading coils from what I have read lately should be placed 2/3 the way down from the center.  May be best to place them 2/3 the way down but locate them at some 1/4 wave length that is equivalent to around the 2/3 point for a two or three band operation, maybe four, thats after I looked into extended antennas.

I have made coaxial traps back in the day when people were just start making them.  I still have 20, 40 and 80m traps that I made.

http://degood.org/coaxtrap/

Jim K9TF
« Last Edit: February 27, 2012, 10:10:14 AM by WA9YSD » Logged
AK4SK
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Posts: 150




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« Reply #6 on: February 27, 2012, 12:37:35 PM »

I had RFI issues with the G5RV due to closeness of proximity. Long Story. It does work well though. Because of the RFI issues I switched to folded dipoles.

By folded dipole do you mean a terminated folded dipole like I mentioned or is a "regular" folded dipole different?

The others antennas you mentioned are inefficient not worth the trouble.

Are you talking about the Cobra or both the Cobra and TFD?
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WA9YSD
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Posts: 138




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« Reply #7 on: February 27, 2012, 05:13:32 PM »

G5RV gave me RFI issues for what ever reason other wise it worked well. I switched to a normal folded dipole which I think worked better on 80 and 40m but could not test them side by side.

I have tried the T2FD cause I built my own with Radio Shack dummy loads in series.  I tried the Radio Waves Variable Terminated Folded dipole. From what WB6BYU had said your 5 DB down by design, he also pointed out to me that by zig zaging wires back and forth, radiated waves will cancel them selfs out and cause a further decrease the effectiveness of power being radiated. With this in mind the Cobra will be less effective than a normal dipole on certain bands.

Jim K9TF
« Last Edit: February 27, 2012, 05:25:30 PM by WA9YSD » Logged
AK4SK
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Posts: 150




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« Reply #8 on: February 28, 2012, 07:07:08 AM »

Thanks Jim.
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W6RMK
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Posts: 648




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« Reply #9 on: February 28, 2012, 07:26:40 AM »

I think the best long term solution for <100 Watts and limited space (but not severely limited) is a remote automatic antenna tuner at the feed point of whatever length wires you can put up.  it doesn't have to be perfectly symmetrical (or even close). There are some "particularly" bad lengths (you don't want the overall length of the antenna to be exactly 1 wavelength), but other than that, you get all band operation, coax feedline, minimum losses in that feedline (since it's always matched).    You can also put multiple wires of multiple lengths up, which will help the pattern on higher bands, if nothing else.

As others have noted, the other choices either have losses in the antenna (and how ever much loss there is in the autotuner, it's probably not 6dB, like the typical Terminated Folded Dipole) or require a low loss feedline and a tuner at the shack.

I've fooled with a variety of "no tune" or "multiband" antennas of one kind or another, and they're fine if you've got more time than money, because you're going to be doing a fair amount of fiddling around to get them to work right.

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AK4SK
Member

Posts: 150




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« Reply #10 on: February 28, 2012, 09:06:13 AM »

Here are some useful articles to start:

http://vk1od.net/antenna/G5RV/index.htm
http://www.w8ji.com/g5rv_facts.htm
http://www.cebik.com/content/a10/wire/g5rv.html
http://www.cebik.com/content/gup/gup6.html

http://vk1od.net/antenna/cobra/index.htm

http://www.cebik.com/content/a10/wire/t2fd.html

The articles at cebik.com are from the late W4RNL, and require free registration to access.
It is worth it, however, as there is a wealth of antenna information on his site.

Depending on your needs, there may be other antennas that work for you.  When trying to
fit in a small space, sometimes you need to custom-build an antenna for the space available
and the combination of bands that is most important to you.  In other cases a straight dipole
(or doublet) fed with twinlead will work - the length isn't critical as long as it is long enough
for the lowest operating frequency.  You may be able to build such an antenna with loading
coils for 80m that allow for a shorter antenna.  Such an antenna needs a tuner.

I read several of those articles. If I understand Cebik correctly one of the antennas I am considering is a terminated wide band folded dipole (or WBFD) and not a T2FD or TFD as I called it. For one, I think the construction of the two is slightly different and also I would be installing mine in an inverted v configuration and not tilted or vertical.

I am looking for my first HF antenna. I guess I should have mentioned that at the start but it's probably obvious. I don't think I can install a vertical because I have basically zero room for radials (you'd have to see how my yard is arranged) and it would be competing for space with a vertical VHF/UHF antenna. I like the WBFD because it works, more or less, on all of the HF bands (although I probably will not be able to install one long enough to work on 160m). Since I have no idea what bands I am going to be using this gives me the opportunity to try them all to one degree or another. I guess I can always specialize from there as I figure things out and get an antenna that is better for what bands I decide I want to use. It is also quite simple to install and use, just elevate the center point, secure the ends, and run 50 ohm coax to the shack and I'm done (I think). No remote tuner, traps, etc. needed.

For my purposes does it sound like an ok idea, or should I still be considering other options?
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WA9YSD
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Posts: 138




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« Reply #11 on: February 28, 2012, 09:39:21 AM »

Take a look at these sites.  Any type of terminated folded dipole has 5 DB loss. As previously pointed out. Mulit-banding a dipole has some sort of loss.  The terminated folded dipole WBFD T2FD VTFD seams to be the bad ones.

www.navymars.org/northeast/reg1/sne/wide_band_folded_dipole.ppt

Put up the G5RV or a trap dipole as an inverted vee.

http://degood.org/coaxtrap/

Regular folded dipoles usually work well on two or 3 bands They will work on others as well.

If you like folded dipoles and want it short try this. The coils have the same amount of wire in each.  Coil placement is around 5 feet from the ends. If you add he length of wire used in the coils and the length of wire for your desired band. You will have to do some pruning. You make your own coils so you can fit the antenna in the space you have.

|-----Coil----------------------------------------------Coil-------|
|                                                                     |
|-----Coil--------------------Feed-------------------Coil-------|


Jim K9TF
« Last Edit: February 28, 2012, 10:22:39 AM by WA9YSD » Logged
WB6BYU
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Posts: 12980




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« Reply #12 on: February 28, 2012, 11:12:48 AM »

Quote from: KK4GDX
...If I understand Cebik correctly one of the antennas I am considering is a terminated wide band folded dipole (or WBFD) and not a T2FD or TFD as I called it...



Don't worry about the difference.  Everyone else calls it a T2FD (Tilted Terminated Folded Dipole)
and W4RNL was just trying to use a more accurate name.  How it is strung up isn't what matters -
the point is that the termination makes the antenna inefficient compared to an equivalent straight
wire fed with open wire line to a tuner.

That's not the end of the world, of course.   With 6dB loss you would have only 25 watts
radiated from a 100 watt transmitter.  I make lots of contacts running just 5 watts, and you
are still getting more output than that.  But you could do better with a more efficient antenna.


Quote

It is also quite simple to install and use, just elevate the center point, secure the ends, and run 50 ohm coax to the shack and I'm done (I think). No remote tuner, traps, etc. needed.

For my purposes does it sound like an ok idea, or should I still be considering other options?


The terminated folded dipole needs to be somewhat longer than 1/2 wavelength or the
efficiency really drops quickly.  The versions around 90 feet long are considerably worse
on 80m than they are on 40m.  If you have room for that, then you have room for a 40m
dipole, which gives you direct coax feed with good efficiency.  Dipoles for the higher
frequency bands are even shorter.

My recommendation is to start with simple half wave dipoles for 40m and 20m on the same
feedpoint, or some other combination of bands that fits your available space.  You can adapt
the 40m dipole to work on 15m, and add a 10m dipole to the mix as you go along.  By using
loading coils or traps you can make dual-band elements, such as 17m + 80m with less
total length.

These are often known as "fan dipoles" because of the way the wires are arranged, but a
true fan dipole is a wideband antenna that can cover, for example, 6 - 30 MHz continuously
using multiple wires of the same length.

This starts simple and allows you to increase the number of bands it covers as you get
more experience with it.  You still have only one coax feed to the shack, and you don't need
a tuner if the wires are adjusted properly.  It's also a lot cheaper to build your own antenna
rather than buying a commercial one.
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AK4SK
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Posts: 150




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« Reply #13 on: February 29, 2012, 06:13:35 AM »

Thanks for the explanation.
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N3OX
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Posts: 8852


WWW

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« Reply #14 on: February 29, 2012, 07:09:49 AM »

That's not the end of the world, of course.   With 6dB loss you would have only 25 watts
radiated from a 100 watt transmitter.  I make lots of contacts running just 5 watts, and you
are still getting more output than that.  But you could do better with a more efficient antenna.

Yeah, and you need to spend money on an amplifier to get back to the equivalent of a barefoot rig running into an efficient antenna.  25W is plenty to make MANY nice contacts, but it's not enough to do everything, and eventually you might want more.  Power is expensive... even 1dB is expensive at some point.

I think lossy matching is silly simply for this reason.  It's not worth the convenience, IMO.




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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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