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Author Topic: Length of ground mounted radials for 20 - 10m fan vertical?  (Read 1366 times)
G7MRV
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« on: April 17, 2013, 04:55:20 PM »

OK so heres the deal - im going back to an idea I had some time back for a fan vertical to provide the extra bands I need for the upcoming special event. An ex-Army Clansman 5.4m fibreglass mast will support the five antenna elements. 20m will be a wire running up the mast (this type of mast is actually designed to be used as a vertical in this way and has holes in the guy rings and tie offs for the purpose), whilst the 17m, 15m, 12m and 10m elements will be supported by horizontal fibreglass rods/tubes mounted in a cross formation at the top of the mast, spacing them a minimum of 1ft/30cm apart, using Dacron line from the element ends to the rods to keep them tight and straight (obviously each element will be a different length, so the line also acts as an insulator and extender), with dacron lines near the bottom to guy pegs to create the necessary bend in the elements so the majority of the element is parallel. Each element will then go to a common feedpoint box where the coax and the radials will connect. A flylead from this box goes to a radial plate, to which a minimum of four, and maximum of about sixteen radials can be connected (i say about, as I cant remember how many it can take!)

Now, the radials will all be laid on the ground, with pegs at the ends to keep them straight and secure. I am aware that in ground proximity they will not be resonant and its a case of the more the merrier. However, will the fact it is a multiband vertical have much influence ont he lengths needed for the radials?

It is my initial thought to make the radials a quarter wave on the lowest operating frequency, which in this case will be 20m, so about 5m.

Will I be fine with radials of this length when operating on the other bands? Also, ive heard that although more is better, up to about 32, that there is an issue with eight radials being of little benefit, even detrimental, over four? Or is this a myth?

How many I can use with it 'on the day' I dont yet know, it will depend on the size of the antenna field I have available to me (probably not huge) and the requirements for other antenna systems and other users.
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K7KBN
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« Reply #1 on: April 17, 2013, 05:36:39 PM »

Start with, say, four radials.  See how it works on each band, check SWR bandwidth, etc.  Then increase the number of radials by a few and see what changed, how much, in what direction, etc.  Keep notes!!

It's called "experimentation".  Results WILL differ from one installation to another even though they "should" be "identical".  See what works and what doesn't.
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73
Pat K7KBN
CWO4 USNR Ret.
G7MRV
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« Reply #2 on: April 17, 2013, 05:56:01 PM »

Start with, say, four radials.  See how it works on each band, check SWR bandwidth, etc.  Then increase the number of radials by a few and see what changed, how much, in what direction, etc.  Keep notes!!

It's called "experimentation".  Results WILL differ from one installation to another even though they "should" be "identical".  See what works and what doesn't.

Yes, I will be doing so. However the actual number used will be something I wont know until I see what can be fitted in at the showground. Hopefully I can do a test run in advance, but its possible I wont know until the day before.

What I need to know is the effect of length. Im pretty sure that so long as the lengths are good for the lowest frequency, they will be ok for the other bands, but i'd like some educated opinion on that before I start cutting
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WB6BYU
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« Reply #3 on: April 17, 2013, 08:53:02 PM »

For radials laying on the ground, a quarter wave on 20m should be fine.  Seems to
me that 1/4 might not actually be an optimum length, as, under some conditions,
the earth loading might make effectively 1/2 wavelength, but for your purposes it
is  good start.  You probably won't notice any difference with reasonable variations
either way from there.

8 radials is a good start, 16 would be somewhat better if you have the time and
space to do so.  You can always crimp 2 or more wires onto each lug or put
multiple lugs under a single bolt if your radial plate doesn't have enough space.
(I use a loop of copper wire and twist all the radials to that - I've never run out
of room.)
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G7MRV
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« Reply #4 on: April 17, 2013, 09:06:32 PM »

Thanks. That sounds good to me.

8-16 should be possible. The number of radials I can fit onto the plate is certainly in that range, but im not sure how many more. I fabricated the plate some time ago and cant remember how many bolts i put into it! It was made for this exact purpose though, and the base of the mast fits within the plate.

The Racal 5.4m Clansman mast has a guying radius of 3m. I always add 2m to that as a safety margin, so radials cut 1/4w on 20m will run just to the safety boundary for this mast, which is ideal.

It was my intention to use my 10m fibreglass fishing pole mast for this, but fabricating the cross member supports for the wires would have been dififcult. The Racal mast I have spare sections of, which I dont mind modifying (one section has already been cut down to provide a mount for a 2m monopole) and can drill to accept a pair of 12mm dia fibreglass tubes to support the wires.

Its essential we have an antenna for the WARC bands at the show, as were on the same weekend as the RSGB NFD contest, so without it we'll get hammered by the contesters.
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RFRY
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Posts: 338


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« Reply #5 on: April 18, 2013, 03:14:33 AM »

Here is a link to charts showing the power lost in radial systems on/in average earth for various numbers and free-space wavelengths of radials vs. the height of the monopole.

The author is Carl E. Smith, who was a very prominent broadcast engineering consultant and founder of the Cleveland Institute of Electronics.

http://www.freeimagehosting.net/mv8px
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VE3WMB
Member

Posts: 290




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« Reply #6 on: April 18, 2013, 07:48:23 AM »

Unless you plan to put down at least 20 radials, making the length of the radials greater that
1/8 wavelength at the lowest operating frequency really doesn't end up buying you much more.

This statement is based on a study done a few years ago (I don't have a reference handy).

Michael VE3WMB


OK so heres the deal - im going back to an idea I had some time back for a fan vertical to provide the extra bands I need for the upcoming special event. An ex-Army Clansman 5.4m fibreglass mast will support the five antenna elements. 20m will be a wire running up the mast (this type of mast is actually designed to be used as a vertical in this way and has holes in the guy rings and tie offs for the purpose), whilst the 17m, 15m, 12m and 10m elements will be supported by horizontal fibreglass rods/tubes mounted in a cross formation at the top of the mast, spacing them a minimum of 1ft/30cm apart, using Dacron line from the element ends to the rods to keep them tight and straight (obviously each element will be a different length, so the line also acts as an insulator and extender), with dacron lines near the bottom to guy pegs to create the necessary bend in the elements so the majority of the element is parallel. Each element will then go to a common feedpoint box where the coax and the radials will connect. A flylead from this box goes to a radial plate, to which a minimum of four, and maximum of about sixteen radials can be connected (i say about, as I cant remember how many it can take!)

Now, the radials will all be laid on the ground, with pegs at the ends to keep them straight and secure. I am aware that in ground proximity they will not be resonant and its a case of the more the merrier. However, will the fact it is a multiband vertical have much influence ont he lengths needed for the radials?

It is my initial thought to make the radials a quarter wave on the lowest operating frequency, which in this case will be 20m, so about 5m.

Will I be fine with radials of this length when operating on the other bands? Also, ive heard that although more is better, up to about 32, that there is an issue with eight radials being of little benefit, even detrimental, over four? Or is this a myth?

How many I can use with it 'on the day' I dont yet know, it will depend on the size of the antenna field I have available to me (probably not huge) and the requirements for other antenna systems and other users.
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