Author Topic: Am I understanding this right?  (Read 9309 times)

Offline Freedom Forged

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Re: Am I understanding this right?
« Reply #30 on: June 02, 2017, 05:18:06 AM »
Karma for both of you for helping me and putting up with my ignorance.  You see, I'm that special kind of dumb you read about in medical books.  :-\
FF

Offline Carl

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Re: Am I understanding this right?
« Reply #31 on: June 02, 2017, 05:36:29 AM »
Karma for both of you for helping me and putting up with my ignorance.  You see, I'm that special kind of dumb you read about in medical books.  :-\
FF

We all are (ignorant of many things) and much of Ham radio can only be learned by experience and while I don't have all of the answers,I can tell others what worked for me and though I am a cheap Ham ,I appreciate good products and have some idea of what works . My biggest gripe is Ham's who pay $300 for a stick of aluminum that 49 cents worth of wire can often out-perform.Advertising hype often outperforms real experience with produces antennas.

Offline Lamewolf

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Re: Am I understanding this right?
« Reply #32 on: June 20, 2017, 08:20:23 AM »
the zep uses a long (take your pick of length) <snip>

The zepp was originally a half wave long endfed wire fed with a quarter wave section of balanced feedline, not just a long random length of wire fed against a counterpoise.

Offline Carl

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Re: Am I understanding this right?
« Reply #33 on: June 20, 2017, 09:39:57 AM »
the zep uses a long (take your pick of length) <snip>

The zepp was originally a half wave long endfed wire fed with a quarter wave section of balanced feedline, not just a long random length of wire fed against a counterpoise.

Karma, you are correct. But now as multiband use is desirable we us a non-resonant length of 1/4 wave plus of the lowest frequency to be used and normally a random length..for me any multiple of 17 feet or a set random length.as the ability to tune many bands is very desirable to some.

Offline Lamewolf

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Re: Am I understanding this right?
« Reply #34 on: June 20, 2017, 09:46:30 AM »
A random wire needs 'balance' and this is why it should be called random wires, the zep uses a long (take your pick of length) against a shorter 17,25,51 foot length of wire and they can be together or apart as a random pair of wires in a dipole configuration.A random wire run against an earth ground can also work,but remember ...half of your power goes to each side and earth does not radiate too well. <snip>

A random wire antenna is by nature an un-balanced antenna, much like a ground plane antenna is un-balanced.  A balanced antenna would be basically a dipole configuration fed with balanced feed line and a balanced AMU (Antenna Matching Unit).  But again, a zepp antenna is a half wave wire fed by a quarter wave balanced feed line, but it is still not a balanced antenna.  The zepp, being and end fed halfwave means that it is fed at a high voltage node, and the quarter wave balanced feedline was designed to convert that to a low impedance and low voltage feed to get the high voltages away from the zeppelin that was filled with explosive hydrogen gas to make it float aloft. If not, the high voltages could arch and ignite any hydrogen leaks from the balloon structure killing everyone on board !

Offline Lamewolf

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Re: Am I understanding this right?
« Reply #35 on: June 20, 2017, 09:59:39 AM »
Karma, you are correct. But now as multiband use is desirable we us a non-resonant length of 1/4 wave plus of the lowest frequency to be used and normally a random length..for me any multiple of 17 feet or a set random length.as the ability to tune many bands is very desirable to some. <snip>

The lengths you mentioned are not 1/4 wave, they are lengths that are non quarter wave or multiples of non quarter waves to help keep the impedance at a more constant level over a wide frequency range which makes them easier to match with a tuner. 

Offline Smurf Hunter

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Re: Am I understanding this right?
« Reply #36 on: June 20, 2017, 10:02:09 AM »
Slightly off topic, but for fixed frequency QRP work - suppose SOTA or other lightweight field setup, would there be any advantage or disadvantage to using a resonant wire length Zepp over a classic dipole?

Offline Carl

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Re: Am I understanding this right?
« Reply #37 on: June 20, 2017, 10:22:07 AM »
Slightly off topic, but for fixed frequency QRP work - suppose SOTA or other lightweight field setup, would there be any advantage or disadvantage to using a resonant wire length Zepp over a classic dipole?

Ease of deployment. Though a dipole offers more capability ..more TX and RX capability ...in my long term use and opinion a 1 to 2 db advantage and this is NOTICEABLE.The ZEPP is best for multi-band and ease of deployment. Dipole or inverted "V" are better on their 'cut to length' band and will often tune other bands ...this is why I still use my 10-20-40 single feed,multiband "V" and added the 51 foot zepp...the "V" beats the zepp EXCEPT when the lower zepp at only 10 feet off the ground allows me to hear stations,and talk to them,that I can't hear through the noise on my higher multiband "V" directional antenna (2 extra DB to North due to orientation. Yard size makes me do this but benefit is most of US is North of me anyway..

A single band antenna also allows you to leave the antenna tuner at home,if not internal, and I like a 40 meter dipole.inv "V" for this as it allows for 40 and 15 meters and also often 10 ,20,17 with a simple internal tuner if multi-band is desired....but now we are losing the gain of a single band antenna when tuning other bands.

Offline Lamewolf

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Re: Am I understanding this right?
« Reply #38 on: June 20, 2017, 11:19:41 AM »

Ease of deployment. Though a dipole offers more capability ..more TX and RX capability <snip>

Sometimes maybe, but not always.  An end fed half wave will radiate exactly like a center fed half wave dipole if deployed the same way.  In other words, if you put them both up at 30' with the wire completely horizontal, the radiation pattern is the same.  Same thing if you put them both up equally as an inverted V.  But again, this is speaking of both being a half wave radiator.  When you feed the half wave from the end, the radiation pattern is the same as it is when center fed, the only thing that changes is the location and impedance of the feed point.  With the center fed half wave, the impedance is around 50 to 75 ohms depending on height above ground.  On the end fed half wave, the feed impedance is on the order of 3000 to 5000 ohms and requires some sort of matching device to convert it down to 50 ohms.  Think of it this way, a half wave antenna always has high "VOLTAGE" and high impedance on the ends, but has high "CURRENT" and low impedance in the center and the high "CURRENT" point (center) is always the point of maximum radiation no mater where its fed from.  Another example is the off center fed dipole, where the impedance is around 200 ohms, but on its fundamental frequency, its a half wave antenna and maximum radiation comes from the center, and not the feedpoint - that is if it has a properly design balun feeding it.

Offline Lamewolf

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Re: Am I understanding this right?
« Reply #39 on: June 21, 2017, 08:26:12 AM »

Offline LodeRunner

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Re: Am I understanding this right?
« Reply #40 on: November 04, 2017, 03:19:05 PM »
For my Zepp antenna if I choose one of the lengths in "green" from this info http://www.hamuniverse.com/randomwireantennalengths.html that will best fit for the room I have available I should be good?  I think I've read so much I'm on information overload. :o

OK, I'm going to jump in here and help out.  But first I'd like to please clarify some terminology for everyone, so that we can focus on ideas  instead of [confusing] language -

A "Dipole" is a 1/2 wave resonant antenna, fed at the Current Loop in the center.  A Dipole is a resonant antenna, so it has a specific frequency for which it is designed.  The feed point at the Current loop will have an impedance (at the design frequency) of between 25j0 ohms and 150j0 ohms, depending on the height of the antenna AGL, and the soil characteristics where the antenna is installed.  A Dipole is sometimes archaically referred to as a "Hertz Antenna" because it was Hertz who first discerned the value of such an antenna configuration.

A Zepplin (Zepp) antenna is a 1/2 wave antenna, fed at a Voltage Loop (which exists at each end).  A zepp is a resonant antenna, so it has a specific frequency for which it is designed.  The 'traditional form' of the Zepp antenna is fed with 1/4 wavelength of open-wire line or "ladder line". This 'Tuned Line' acts as an impedance transformer (at the design frequency) to transform the High-Z (typically near 3000 ohms) of the Voltage Loop at the end of the antenna down to an impedance low enough to be matched to the transmitter - usually in the range of 30~300 ohms depending on the characteristics of the Tuned Line (matching section).

An EFHW (End Fed Half Wave) is a Zepp antenna which doesn't use the traditional 1/4 wl Tuned Line impedance transformer, but uses instead some form of "lumped matching" such as a parallel-resonant tuned circuit, or a "step up transformer" wound on a high permeability ferite toroid or rod.

A "Doublet" is a wire antenna, fed at or near the center, which may or may not be a resonant half wave.  A dipole is always a doublet, but a doublet is not always a dipole.  A G5RV is a "Multi-band Doublet antenna" but is -not- a "Dipole".

An "Off-Center Fed Dipole"(sic) is really a Doublet.  It's not a Dipole because it's not fed at the Current Loop in the center.  And the main reason for feeding it off center is to make it a multi-band antenna - so it won't be 1/2 wavelength long on any but the lowest band for which it is designed, so again, not a "Dipole".

A Random Wire is an End-Fed antenna which is not intended to be any multiple of 1/2 wavelength on the band(s) for which it is used.  So a Random Wire is and end-fed antenna which is -Not- a Zepp.

-----
By way of apology for treating everyone like they're in radio kindergarten, let me say that on so many forums I have encountered language so confusing that it's almost impossible to share information.  It gets very frustrating, so I just want to make sure everyone understands what I'm saying and why I sometimes nit-pick about the terminology we use.  I'm typically worth putting up with in this regard.  Honest  ;P
-----

Here's why the differences in terminology actually matter to us:

The length of a Random Wire is chosen so as NOT to be a Zepp on any of the desired band(s) of operation, because the impedance of a Zepp is far to high for a typical antenna tuner (transmatch) to work with.  This is why the chart you linked is helpful - the publisher has done the math to help you rule out the lengths where a wire will behave as a Zepp on one (or more) bands. 
FWIW, the "Sweet Spots" for a Random Wire antenna are all near odd multiples of 1/8th wavelength, i.e. 1/8, 3/8, 5/8, 7/8, etc.  The reason is because the resistive part of the impedance will be in the range of 200 to 500 ohms, with only a moderate amount of reactance which the antenna tuner can match (make to appear resonant) over a large span of frequencies.

One thing which the creator of that chart didn't mention is the fact that the resonant lengths (to avoid) car vary substantially with the height at which the antenna is deployed, and the soil characteristics under and around the antenna.  Here are two rules of thumb that always apply to the antenna types discussed above:
The lower you hang a wire, the shorter it will be to achieve resonance on a given frequency.
The better the soil conductivity is, the shorter the wire will be to achieve resonance on a given frequency.
These apply to dipoles, zepps, random wires, "OCF Dipoles" (sic), and pretty much any transmitting antenna.  These rules-of-thumb do NOT apply to Beverage receiving antennas, or to other novel antennas such as pennants, flags, "magnetic loops", and "magnetic slots", among others.

OK, back on target now -
From the looks of the chart you linked to, he is assuming that the "resonant" lengths to avoid are all based upon ~1/2 wavelength as the working height AGL.  This is an error, because the 1/2wl height is very different for each band. However, this is a small error in the given context, and shouldn't hurt your results by using it, as-is.

SO - the chart will be "close enough" for wires hung horizontally at 30~40 feet AGL for most types of soil. If your wire is hung much less than 35 feet AGL you may (probably) need to shorten the wire just a bit (figure 5% or so), and if it is installed much higher than 35 feet AGL you may need to make it a bit longer.  Deploying such a random wire as a "sloper" or "Inverted Vee" may also change the optimum lengths, sometimes in unpredictable ways (unless you model the antenna first, using known-good values for *your* soil characteristics).

ALSO - EFHWs and End-Fed wires *always* require a good ground or counterpoise for proper operation.  If you omit the ground/counterpoise, or allow your coax and station equipment to become the counterpoise for your antenna, then you will likely suffer some or all of the following - poor radiating efficiency of the antenna; difficulty in achieving (or maintaining) a good match to the antenna with your ATU; increased sensitivity to received noise (interference) from computer and appliances in your home and shack; RF-hot mic and/or equipment chassis while transmitting; irregular operation (rig locks up or otherwise misbehaves while transmitting); moderate to heavy distortion of transmitted audio, and RF spurs outside the intended transmit bandwidth (interference to other stations).

Cheers