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Author Topic: Which Balun for G5RV and LDG AT-7000  (Read 2779 times)
VK1OD
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« Reply #15 on: November 14, 2009, 11:08:43 AM »

Foldback is a term used to describe the behaviour of the protection circuit on some power supplies where the current limit is effectively reduced under overload.

This is to mean that if for example such a power supply was rated at 20A, when that current is exceeded and the output voltage drops, the current supplied into the load may be reduced to well below 20A, perhaps more like 2A into a short circuit for example.

The reason for doing this is that an ordinary current limited power supply would dissipate a very high amount of heat if it delivered its current limit into a short circuit.

If you were to plot V vs I for load resistance decreasing from infinity to zero, the characteristic appears to 'fold back'.

The circuit techniques are well know and described in many if not most linear regulator chip datasheets.

I am not aware of this type of fold back mechanism being built into the protection circuits of common ham transmitters. Typically, part of the protection circuit just reduces IF gain so that the reflected power does not exceed a certain quantity. There is no 'folding' back and I can't quite understand the growing trend to label such circuits 'fold back', the term is probably misused in this application.

Owen
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K0BG
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« Reply #16 on: November 14, 2009, 11:18:15 AM »

Owen, it's sort of like using counterpoise instead of ground plane. Different animal, but you wouldn't think so from this site!

Alan, KØBG
www.k0bg.com
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VK1OD
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« Reply #17 on: November 14, 2009, 11:22:38 AM »

Chris,

Here is a different perspective.

The G5RV antenna system is a dipole and some type of feedline system.

If the purpose of the feedline (be it open wire, coax, or some combination) is solely to carry energy from the transmitter to the dipole centre *without* contributing to radiation of itself, then the feedline must carry only differential current, ie *no common mode current*.

G5RV described a feedline configuration using a cascade of openwire line and coax with the tx end of the shield grounded, and in his first article he recommended against a balun. That configuration *will* encourage common mode current.

If you want to minimise common mode current and the associated adverse effects, you should use an effective current balun as a matter of course to use the system with an unbalanced ATU.

Owen
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VK1OD
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« Reply #18 on: November 14, 2009, 11:31:20 AM »

Chris,

Regarding the AM setup of the '706.

The PA in the 706 is capable of 100W PEP on HF.

The PEP of a fully modulated (ie m=100%) AM wave is four times its carrier power, so the maximum fully modulated carrier wave that the '706 PA can handle is 25W.

In their wisdom, Icom choose to configure the radio to operate at 40W carrier on AM, so your peak modulation is limited to 58%. Other manufacturers do it differently.

The correct behaviour of the '706 on AM is maximum of 40W carrier.

RF power measurement terms are not widely well understood in our community. The article at http://vk1od.net/measurement/RfPowerTerms/RfPowerTerms.htm might help you. The example waveform is modulated to 60%, so you can see visually, the modulation underachievement of Icom's AM setup.

Owen
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CHRISDX
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« Reply #19 on: November 14, 2009, 11:40:00 AM »

Hi Owen,

So far there seem to be different opinions. It seems that there is no clear answer on whether to run a balun, or which one.

Common mode current seems the issue.  I've seen mention of coiling the coax, and other mention that this is ineffective. There was also different approached, how many winds and around what diameter, and where to make the coil. At the junction to the ladder line seems the place.

There was also info about applying a choke to the coax. Not sure what choke or where to get one, I'll read back and re-read it.

So start with this, what is your exact proposed optimization and with what items, placed where?  Is your proposed optimization to reduce "common mode current" and why would I want that reduced, because the "unbalanced auto tuner" would not 'like' it? if so, what does that mean, does it affect the output signal from my station or impact the life of the tuner, or both, or something else?

-chris
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VK1OD
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« Reply #20 on: November 14, 2009, 11:43:03 AM »

Chris,

I have written articles at:

http://www.vk1od.net/antenna/G5RV/index.htm

http://www.vk1od.net/antenna/G5RV/optimising.htm

that may help you.

Owen
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CHRISDX
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« Reply #21 on: November 14, 2009, 11:56:53 AM »

OK, I read the link by VK10D Owen and am reading the others.

http://www.vk1od.net/antenna/G5RV/index.htm

Question:  how do I determine the impedence and if necessary, other parameters of the antenna as installed.  For example, I have a digital multimeter, do I simply disconnect the 239 at the end of the ladder line section of my G5RV and test ohms?

Sorry but I'm kind of new and have to start with the basics!

There is also a mention of "voltages at remote tuner". While I dont have a rmeote tuner configuration, where is the voltage (before or after the remote tuner), when does it happen (when trans,itting?) and how is it measured?
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VK1OD
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« Reply #22 on: November 14, 2009, 12:17:03 PM »

Chris,

The first article is a detailed analysis of various ways of feeding a G5RV.

The message you can take away is that there *are* different ways to do it, and the different methods have significantly different performance. You will note that the methods that use coax at high VSWR are relatively poorer.

In your case, I would use a 1:1 current balun designed for use with an ATU.

LDG are not the only manufacturers of baluns, and I would suggest you look around. Balun Designs is one manufacturer that provides detailed information on at least some of their baluns.

Alternatively, you could build one. There are plenty of articles out there, but that is no assurance that they are good designs.

I have a current project for a 'general purpose' 1:1 current balun to use with a tuner, but it is stalled with moving house. The design is discussed at http://vk1od.net/blog/?p=581 which might give you some ideas in what to look for. (The core used is in American speak, a FT240-52 or -K.)

Owen
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K1BXI
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Posts: 812




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« Reply #23 on: November 14, 2009, 02:19:16 PM »

"Question: how do I determine the impedence and if necessary, other parameters of the antenna as installed. For example, I have a digital multimeter, do I simply disconnect the 239 at the end of the ladder line section of my G5RV and test ohms?"
========================================================
Chris.........Impedance is a measure of AC resistance. Your multimeter measures DC resistance. Since all RF is AC, to do those measurements an antenna analyzer such as the MFJ-259B would be the easiest way to do it.

The voltages mentioned are also AC and are RF voltages developed by the transmitter and appear on both sides of the tuner. You might do well to purchase a handbook which would help explain all this "magic stuff".

As your finding out, your amateur license is really "a license to learn".


John
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KI4SDY
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Posts: 1452




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« Reply #24 on: November 15, 2009, 11:20:28 AM »

You need to add a 1/1 balun at the end of the G5RV ladder line. However, you may find the same result after you add the balun because most manual tuners have a wider tuning range than auto tuners.
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W5DXP
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« Reply #25 on: November 15, 2009, 02:49:47 PM »

My commercial G5RV came with a 1:1 choke (w2du balun) installed at the coax/twinlead junction. That is good engineering practice. Will it do some good? Maybe. Will it do any harm. NO!

If you want to do serious antenna work, an antenna analyzer, like the MFJ-259B, is a good idea.

"Foldback" has become part of ham radio lingo as meaning any reduction in output power automatically caused by circuitry internal to the transmitter - common usage - not exactly technically correct - beats saying, "Reduces transmit power to a safe level caused by the current or voltage or reflected power being too high".
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
OLDFART13
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« Reply #26 on: November 27, 2009, 09:56:54 AM »

Try www.balundesigns.com
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