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Author Topic: S9 31 FOOT VERTICAL  (Read 23172 times)

Posts: 12621

« Reply #30 on: November 18, 2012, 05:45:43 PM »

30 ft of conductor is close to 1/4WL on 40m. 1/4WL is known to be an efficient vertical radiator.

Bit short for 40 by about 3 feet so not a good match.

30 ft of conductor is close to 1/2WL on 20m. 1/2WL is known to be an efficient vertical radiator.

About 3 feet short for a 1/2 wave on 20 too

30 ft of conductor is close to 5/8WL on 17m. 5/8WL is known to be an efficient vertical radiator.

and 2.5 feet short for 17m too

All one needs for efficient multiband performance is a good power delivery system which includes a conjugate match and low-loss transmission line.

Point is it does not really do anything really well and is a compromise at best. A resonate antenna will outperform it.

Ham since 1969....  Old School 20wpm REAL Extra Class..

Posts: 17479

« Reply #31 on: November 18, 2012, 07:28:45 PM »

Quote from: W8JX

Point is it does not really do anything really well and is a compromise at best. A resonate antenna will outperform it.

You may be able to invent circumstances where it would, but simply being resonant
doesn't imply anything for antenna performance or efficiency.  If it were a resonant
half wave on 20m instead of being a bit short, it would probably work a bit WORSE
in many cases.

Here is an example:  using a 31' vertical on 20m with 75' of RG-213 to a tuner in
the shack and no balun at the feedpoint, maximum radiation is -0.05dBi at 20 degrees.
The coax loss is 5.335dB, so taking that into account the radiated signal is -5.38dB.

If we lengthen the antenna to 33.48' is is now resonant.  Radiated signal increases
to +0.19dB, an improvement of 0.24dB.  But even though the antenna is resonant
the coax losses have increased to 5.837dB, for a radiated strength of -5.64dB.
Your signal DROPPED by about 0.2dB when you made the antenna resonant.

As you increase the length further towards 5/8 wave (which isn't a resonant length)
the antenna gain increases and the angle of maximum radiation lowers a bit.  In this
situation the difference among 31', 33.5' (resonant), or 37' is a fraction of a dB, and
the resonant case does NOT give the best performance, regardless of whether or
not you include the coax losses.  If you don't care about a couple tenths of a dB,
you can say that all three of those lengths will work about the same, but it certainly
is NOT the case that the resonant antenna outperforms the others.

What makes the biggest difference is the coax loss - around 5 to 6dB in this case (about
half that with a perfect 4 : 1 transformer.)  The reason that the antenna is 31' instead
of a resonant length is to keep the voltage down somewhat so it can be matched with
a relay-switched autotuner, which is the most effective way to feed it on most bands.  
In that case, lengths that are longer than resonance will give a bit more radiation than
resonant lengths.

So, yes, the 31' vertical is a compromise, but so is any other antenna, regardless of
size:  you pick a set of properties that meet your needs, even if they are different than
those that others would pick.  The behavior is relatively predictable, rather than being
contrary to the laws of physics.

Posts: 875

« Reply #32 on: November 19, 2012, 02:14:05 AM »

BYU - right on the money!

Even a dipole/yagi/quad is a compromise antenna except on its design frequency and perhaps 15m for a 40m dipole.
As soon as you go multiband - all bets are off for almost all antenna's.
Even Log-Periodic antenna's are compromises in that not all elements are involved on every frequency.
Life is daily full of compromises, and antennas which are not shape shifters have the same limitations.

I used a fixed vertical for many years with great results - now I am using a magloop.
Both are compromise antenna's, but like those of us who have to live with others, it mostly works out.
My current whacky project is getting a thin copper wire to climb up 23 feet inside a fibreglass tube.
A poor mans version of the smallIR vertical.
We will see how that turns out - and it will be educational to compare its performance with my old fixed vertical.

I subscribe to the Missouri slogan of "show me" when it comes to antenna's.
Most of the time it is me showing myself how something works, but the idea is the same.

Happy experimenting,

73 - Rob

Posts: 4311


« Reply #33 on: November 19, 2012, 06:55:49 AM »

Point is it does not really do anything really well and is a compromise at best. A resonate antenna will outperform it.

Why does a non-resonant 5/8WL vertical outerform a resonant 1/4WL vertical?

According to EZNEC, a non-resonant 31' vertical used on 17m, at an elevation angle of 10 degrees, has about 4dB advantage over a resonant 33' 1/4WL vertical used on 40m. All the 31' vertical needs is an auto-tuner at the base or a simple coil-matching technique as is used in 5/8WL verticals (also non-resonant).

The most amazing 10m vertical performance I have ever seen was a 22' non-resonant vertical with four 22' elevated radials slanted down at 45 degrees and fed with an SG-230. EZNEC sez it has a gain of 4.2 dBi @ 10 deg. TOA.

73, Cecil,

Posts: 2276

« Reply #34 on: November 19, 2012, 08:56:31 AM »

Yeah Cecil I agree. I found that when we use three lengths 22ft 44ft 66ft we can choose the better length to better serve dx pursuits on one of our allocated HF ham bands In my case I used wire and worked out the hoisting details to include a commonality for quick change rather than loading coils etc.

I for example with this revelation about three years ago designed an auto tuner and balun that would handle the 1/2 wave pressure regardless of which length chosen and frequency within the range of 1/4 to 5/8 wave radiator respectively.
The primary challenge was eliminating the feedline while keeping a sensitive remote tuner protected by lightning, since eliminating feedline generally places the tuner and balun network at the antenna terminals. I found a unique solution where I place the network of auto tuner and Balun on a window sill and construct radials two per band 10m pair 12m pair up to 75m tune them then press the better length radiator pushing against the pair that is pertinent and already in place. The work of improving an Miltie band radial set is our current challenge holding the three better lengths unchanged as we search for an better Miltie band radial sets of pairs of tuned elevated sets.

The construction of the radial sets simply included sloping downward to isolated from dc path to earth surface using a series of Teflon anchors.

The vertical wires can be disconnected from the system and switched to ground outside for better lightning arresting design etc.

So yes all the factors of individual specifics respective to feedpoint base height, slope angle of elevated radials  and DC isolation of the radials sets can be modeled for estimates of performance prior to design.

One of the first steps to design is considering we can get the better performances when we utilize 1/4 wave to 5/8 wave limits on frequency applied to one of the three length's. Mine cost $20 worth of wire and a rope over a tall tree as a hoist the vertical angle was almost straight up but perhaps a few degrees.The Black color insulated#14 stranded copper elevated radials were parallel to the back wall of the house and positioned opposite each other very inconspicuous from the 5ft feedpoint the network was weather proof and a wander lead was used to place the vertical wire/s to a ground rod outside.

Performance was excellent.
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