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Author Topic: 1/4 wave vs 5/8 wave vertical  (Read 2433 times)
KR4TH
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Posts: 43




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« on: April 26, 2013, 01:54:00 PM »

I will be relocating within a year and considering which vertical to use. (I am in a location where I can only use a 40 dipole for all bands) I have a butternut HF6 which is only 1/4 wave on higher bands.  I want to be able work dx on 10, 15, and maybe 20,  Have you compared or experienced any advantage of 5/8 over a 1/4 wave vertical (or multiband vertical)?  The tuning on the butternut seems less complicated. 



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K0ZN
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« Reply #1 on: April 26, 2013, 09:04:04 PM »

Obviously, theory says a 5/8ths has a little gain, but in the real world, unless you have a large, extensive radial system (for optimum results/gain, you need
radials approximately the length of the 5/8ths radiator!) you won't get all of the theoretical gain out of a 5/8ths. Also, you face the issue that the antenna
will only be 5/8ths on one band.....unless you mechanically adjust it!

1/4 wave verticals (or ground plane antennas) are typically simpler to make and tune (a 5/8ths has an "odd ball" and complex feed point impedance) since
they offer a decent match directly to 50 ohm coax. Also, a 1/4 wave antenna only requires 1/4 wave radials.

1/4 wave verticals can perform very well..... IF ......you have a good, low loss (i.e. "big") radial system because the ground radial system is the other HALF
of that antenna.

If you are wanting to optimize the antenna for one band, then a 5/8th is a much easier project.

Just remember, that with EITHER a 1/4 or 5/8ths length radiator, essentially 50% of the antenna SYSTEM is the Ground Radial system! (Unless you
go to elevated radials or a ground plane antenna.)  A small or inadequate radial system will reduce the effectiveness of any ground mounted vertical.

73,  K0ZN
« Last Edit: April 26, 2013, 09:06:14 PM by K0ZN » Logged
K3VAT
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Posts: 701




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« Reply #2 on: April 27, 2013, 02:13:25 AM »

I will be relocating within a year and considering which vertical to use. (I am in a location where I can only use a 40 dipole for all bands) I have a butternut HF6 which is only 1/4 wave on higher bands.  I want to be able work dx on 10, 15, and maybe 20,  Have you compared or experienced any advantage of 5/8 over a 1/4 wave vertical (or multiband vertical)?  The tuning on the butternut seems less complicated. 

KR4TH: K0ZN summaries the issues nicely.  For more detailed discussions you can search the forum archives - there is dozens of postings and hundreds of comments on this popular subject.

GL, 73, Rich, K3VAT
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RFRY
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« Reply #3 on: April 28, 2013, 03:40:06 AM »

The r-f resistance of a given set of radial wires symmetrically buried around the base of a monopole is a function of their physical free-space wavelength, the number of wires used, and the conductivity of the earth in which they are buried, at the operating frequency.

The graphic linked below shows system efficiencies for various configurations, for earth conductivity at the antenna site as low as 2 mS/m.

As the charts show for a given set of buried radials, antenna system efficiency varies with the height of the monopole.  This is due mostly to the lower radiation resistance (Rr) of shorter monopoles.  Antenna system radiation efficiency = Rr / (Rr + System Loss).  System loss includes the losses in matching/loading networks and the r-f ground system.

The r-f loss in a radial system using any set of buried wires essentially is the same for a 1/4-wave monopole as for a 5/8-wave monopole.  System radiation efficiency for a 1/2-wave and  5/8-wave monopole are slightly better than for a 1/4-wave monopole because the Rr of the longer monopoles is higher.  But the major difference in the low-angle fields that 1/2-wave and 5/8-wave monopoles radiate is due to the shapes of their elevation patterns -- which have somewhat more gain at those lower angles.

The graphics also disprove the common belief that buried radials need be only as long as the height of the monopole.  Take the case of a 70-deg monopole used with 60 x 0.2-wave (72-deg) radials compared to using it with 60 x 0.4-wave (144-deg) radials.  The system using the longer radials is almost 2-1/2 times more efficient.  If 120 radials are used the difference between them is more than 12 times, for the longer set.

http://i62.photobucket.com/albums/h85/rfry-100/GndSystemLosses_zps0b36c41e.jpg

R. Fry
« Last Edit: April 28, 2013, 03:51:01 AM by RFRY » Logged
KC4MOP
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Posts: 729




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« Reply #4 on: April 28, 2013, 04:34:53 AM »

5/8 wave vertical gives the lower angle of radiation for long distance or DX. Take off angle about 12 degrees. 5/8 wave for a typical ham op becomes manageable 20M up. 40M is an 80 foot tower. Not too many Ham ops can deal with that kind of tower/mast.
Google is getting pretty good with Amateur radio stuff and antennas. Do some research.
As with radials; you always run the longest wires , and as many as you can. (30-60 seems good) If you are willing to put a little more money into radials, then buy vinyl covered, welded fencing. And lay out about 8 of those around the base of the vertical. Get the 4 foot wide 50 feet long and bond together with 2 inch copper strap to the braid of your coax and ground rod. A lot of work and $$ but makes my 40/15/17 meter vert shine. I ran a separate wire extended from the 34 foot 40/15M mast and I have a 5/8 wave 17M antenna. With antenna switcher on the ground and a series coil for matching the 17M ant wire. It's really sweet.
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K3VAT
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Posts: 701




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« Reply #5 on: April 28, 2013, 05:10:23 AM »

The r-f resistance of a given set of radial wires symmetrically buried around the base of a monopole is a function of their physical free-space wavelength, the number of wires used, and the conductivity of the earth in which they are buried, at the operating frequency.

The graphic linked below shows system efficiencies for various configurations, for earth conductivity at the antenna site as low as 2 mS/m.

As the charts show for a given set of buried radials, antenna system efficiency varies with the height of the monopole.  This is due mostly to the lower radiation resistance (Rr) of shorter monopoles.  Antenna system radiation efficiency = Rr / (Rr + System Loss).  System loss includes the losses in matching/loading networks and the r-f ground system.

The r-f loss in a radial system using any set of buried wires essentially is the same for a 1/4-wave monopole as for a 5/8-wave monopole.  System radiation efficiency for a 1/2-wave and  5/8-wave monopole are slightly better than for a 1/4-wave monopole because the Rr of the longer monopoles is higher.  But the major difference in the low-angle fields that 1/2-wave and 5/8-wave monopoles radiate is due to the shapes of their elevation patterns -- which have somewhat more gain at those lower angles.

The graphics also disprove the common belief that buried radials need be only as long as the height of the monopole.  Take the case of a 70-deg monopole used with 60 x 0.2-wave (72-deg) radials compared to using it with 60 x 0.4-wave (144-deg) radials.  The system using the longer radials is almost 2-1/2 times more efficient.  If 120 radials are used the difference between them is more than 12 times, for the longer set.

http://i62.photobucket.com/albums/h85/rfry-100/GndSystemLosses_zps0b36c41e.jpg

R. Fry

Thank you for your posting.  However, I'm having trouble with the calculation in your last paragraph, and would like some assistance.

For a 70 degree H shortened vertical I examined the graphic (from your link) for 60 radials and from the chart is appears that when using .2 wavelength long radials the percent loss in the ground system equals ~ 26%, correct?  When using .4 wavelength long radials the percent loss in the ground system ~ 11%, correct?  Doesn’t that equate that the .2 wavelength long radial system has a little over twice the associated loss when compared to the .4 wavelength long radial system?

A 70% vertical has a radiation resistance ~ 22 ohms, per figure 9-7 from ON4UN’s text (page 9-5).

If we assign a 5 ohm loss to the shortened vertical described by Smith for the system using .4 wavelength long radials, and using a RR = 22 ohms, the antenna system efficiency is then 22/(22+5) or about 81%.  If we then use a 11 ohm loss (this is a bit more than twice the loss calculated from above) for the system using .2 wavelength long radials, the system efficiency is then 22/(22+11) or about 67%.

Comparing the efficiencies of these two systems: 81% vs 67%, it appears that the using .2 wavelength long radials drops system efficiency only about 20%, correct?

Assigning higher loss to the above, say a loss of 10 ohms for the system using .4 wavelength long radials, then the ~ loss for the .2 wavelength long system equals approx. 22 ohms.  Using the same efficiency formula, the .4 system ~ 70% and the .2 system exactly = 50% efficiency.  The drop in system efficiency for this example (70% vs 50%) is more pronounced at ~ nearly 30%, correct?

These calculations seem to be markedly different from those appearing in your posting (last 2 sentences).

73, Rich, K3VAT
« Last Edit: April 28, 2013, 05:40:04 AM by K3VAT » Logged
RFRY
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« Reply #6 on: April 28, 2013, 06:54:45 AM »

Thank you for your posting.  However, I'm having trouble with the calculation in your last paragraph, and would like some assistance. etc

The vertical scales in Carl Smith's graphics are logarithmic, so the system loss for 60 x 144-deg radials with a 70-deg monopole is barely greater than 10%, not ~11%.  But yes, my post should have have compared ERPs.  Thanks for picking up on that.

Based on Smith's graphs, for 1 kW of matched power applied to the feedpoint of a 70-deg monopole using 60 buried radials, the system would radiate about 900 watts when using 144-deg radials, and about 740 watts when using 72-deg radials.  The improvement in ERP when going from 72-deg to 144-deg radials in this comparison is almost 22%

R. Fry
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KR4TH
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Posts: 43




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« Reply #7 on: April 29, 2013, 04:31:48 AM »

Thank you for your detailed and informative responses.  They are very helpful.
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RFRY
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« Reply #8 on: April 29, 2013, 05:59:09 AM »

Followup...

Below is a link to a comparison of the elevation patterns of monopoles ranging from 90 to 225 degrees in height.

It can be seen that the improvement in low-angle gain for taller monopoles is the result of the shape of their elevation patterns.

A 225-deg (5/8-wave) monopole has the highest gain in/toward the horizontal plane, but also has a high-angle lobe that may be undesired even in ham radio service.

High-power AM broadcast stations such as WJR and WGN typically use 195-deg monopoles, because the nighttime skywave from the high-angle lobe from a 225-deg monopole can interfere with the groundwave it radiates in areas where the two signals have about equal strength at a receiver.

http://i62.photobucket.com/albums/h85/rfry-100/MWElPatComparison.jpg
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