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Author Topic: Best 30 Meter Antenna  (Read 2030 times)
NK7Z
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Posts: 756


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« on: March 21, 2013, 09:06:41 AM »

Hi,

I have a largish lot, some RFI, and a need to operate 30 Meters...  Been thinking of a full sized vertical, (with radial field), but the RFI issue slows me down there...  Also been thinking of a loop...  I have zero experience with loops...  Has anyone used both, and if so how are they for DX?
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Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
WB6BYU
Member

Posts: 13143




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« Reply #1 on: March 21, 2013, 09:22:22 AM »

RFI is a separate issue from antenna design.  Don't confuse the two.


If you are limited to a relatively low height then vertical polarization may be
better on 30m than horizontal polarization, depending on your ground
conductivity.  (The trade-off point likely is around 1/4 wavelengths: lower
for poor soil, higher for good soil.)

For vertical polarization either a quarter wave vertical or a vertical loop
fed for vertical polarization would work:  the loop takes up more horizontal
space but doesn't require radials.  A common implementation is to use a
delta loop with the point up, fed 1/4 wave down one side from the top
corner.  Another option would be to make an elevated vertical (a wire
hanging from a tree branch perhaps) with two elevated radials.

The performance will be about the same for all these options:  the delta
loop will have a slight peak broadside to the loop and a dip in signal
strength off the ends, but it might not be enough to notice.  Mostly it
depends on what best suits your available supports and materials and
doesn't get in the way of other uses in your yard.

A horizontal full wave loop is not a good idea because high angles
rarely provide useful propagation on 30m.
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NK7Z
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Posts: 756


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« Reply #2 on: March 21, 2013, 11:35:51 AM »

RFI is a separate issue from antenna design.  Don't confuse the two.


If you are limited to a relatively low height then vertical polarization may be
better on 30m than horizontal polarization, depending on your ground
conductivity.  (The trade-off point likely is around 1/4 wavelengths: lower
for poor soil, higher for good soil.)

For vertical polarization either a quarter wave vertical or a vertical loop
fed for vertical polarization would work:  the loop takes up more horizontal
space but doesn't require radials.  A common implementation is to use a
delta loop with the point up, fed 1/4 wave down one side from the top
corner.  Another option would be to make an elevated vertical (a wire
hanging from a tree branch perhaps) with two elevated radials.

The performance will be about the same for all these options:  the delta
loop will have a slight peak broadside to the loop and a dip in signal
strength off the ends, but it might not be enough to notice.  Mostly it
depends on what best suits your available supports and materials and
doesn't get in the way of other uses in your yard.

A horizontal full wave loop is not a good idea because high angles
rarely provide useful propagation on 30m.

Hello and thank you for your help...  I mentioned RFI because I believe verticals are more susceptible to RFI as opposed to a loop even if it the loop is vertically polarized.  Is this a true belief on my part? 

Given two antennas, one a vertical, and the other a loop fed such that it is vertical polarization would there be any difference in the RFI levels fed to the receiver if both antenna were in the same location, and assuming the loop is omnidirectional? i.e. does the loop pick up more RFI than a vertical or vertical dipole?

I have no experience with loops, (lots with verticals), so I don't know if vertically polarized loops are as sensitive to RFI as normal verticals, or vertical dipoles... 

Actually writing the questions has helped me form more exactly in my mind what I need answered, so thank you again!! 

Part of my belief that the loop is quieter is based on a lot of "stuff" I have read on the net about how loops are quieter because they are closed loops...  I am attempting to get a really decent set of answers to this with some tech reference as to why a loop would be quieter...  Again, thank you for your help here!

My RFI source is such that I can arrange a null to point at it if I use a loop, where a vertical will have no null... 
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Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
WB6BYU
Member

Posts: 13143




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« Reply #3 on: March 21, 2013, 12:58:34 PM »

OK, RFI on receive, not home electronics going haywire when you transmit.

You're right, you hear a lot about loops being quieter than verticals, but
noise (unless conducted due to common mode current) is an RF signal just
like the one you are trying to receive, and the antenna can't tell the
difference between them.  W8JI's page on noise may lead you to some
useful information:
http://www.w8ji.com/noise.htm

To get a proper null you would need a pair of verticals spaced 1/2 wavelength,
which you can't get with a full-wave vertical loop unless it is very low and wide. 
You get a little with quarter wave spacing, but probably not enough to make a
difference.

One solution is to use a second vertical and phase they:  you can get a null
with shorter spacings when you can adjust the relative phase of the two
antennas rather than relying on the phase shift around the loop.  The null
needn't be exactly off the back or side:  with a second moveable element
and adjustable phasing you can maximize radiation in several different
directions as you choose while still placing a null at the noise source.  Or
two vertical elements in a cardiod pattern will give something close to
an omnidirectional pattern with a sharp null at the offending source.  It's
also possible to get a sharp rear null from a 2-element yagi using a close-
spaced director, and I'd have to model it to see if that would work with
a parasitic vertical element as well (roughly 5' spacing I think.)
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AD6KA
Member

Posts: 2237




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« Reply #4 on: March 21, 2013, 01:03:13 PM »

Quote
My RFI source is such that I can arrange a null to point
at it if I use a loop

Just an aside, but have you tried to determine
the source of the noise? (what you call RFI)
Is it man made?
(Loose or damaged power line insulator, etc)
Or is it just to generic to tell?

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VE3WMB
Member

Posts: 286




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« Reply #5 on: March 21, 2013, 08:26:32 PM »

You might also look at a half square as an option. This would be equivalent to
having phased verticals for 30m and will give you some gain and a bit of directivity
over a single vertical.

Try this article for starters :

http://rudys.typepad.com/ant/files/antenna_halfsquare_array.pdf

Michael VE3WMB


Hi,

I have a largish lot, some RFI, and a need to operate 30 Meters...  Been thinking of a full sized vertical, (with radial field), but the RFI issue slows me down there...  Also been thinking of a loop...  I have zero experience with loops...  Has anyone used both, and if so how are they for DX?
Logged
W5DXP
Member

Posts: 3553


WWW

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« Reply #6 on: March 22, 2013, 05:45:32 AM »

... but noise (unless conducted due to common mode current) is an RF signal just
like the one you are trying to receive, and the antenna can't tell the difference between them.

True for sky wave noise but local noise can be polarized, sometimes mostly vertically polarized. At my past QTH, the vertical noise from a power pole ground wire was 2 S-units higher on my vertical than on my horizontal dipole rendering the vertical virtually unusable for receiving.
Logged

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.
NK7Z
Member

Posts: 756


WWW

Ignore
« Reply #7 on: March 22, 2013, 06:28:01 AM »

To get a proper null you would need a pair of verticals spaced 1/2 wavelength,
which you can't get with a full-wave vertical loop unless it is very low and wide. 
You get a little with quarter wave spacing, but probably not enough to make a
difference.

One solution is to use a second vertical and phase they:  you can get a null
with shorter spacings when you can adjust the relative phase of the two
antennas rather than relying on the phase shift around the loop.  The null
needn't be exactly off the back or side:  with a second moveable element
and adjustable phasing you can maximize radiation in several different
directions as you choose while still placing a null at the noise source.  Or
two vertical elements in a cardiod pattern will give something close to
an omnidirectional pattern with a sharp null at the offending source.  It's
also possible to get a sharp rear null from a 2-element yagi using a close-
spaced director, and I'd have to model it to see if that would work with
a parasitic vertical element as well (roughly 5' spacing I think.)

Your comment got me to thinking...  I am not sure why I did not think of this before...  I can put up a bobtail curtain array for 30 M, pointing north...  The geometry of the lot is such that the null would be pointed directly at my RFI source as well...  THANK YOU !  That antenna I have experience with, and know how it works...

I do want to play with a Delta Loop though...  Smiley

Logged

Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
NK7Z
Member

Posts: 756


WWW

Ignore
« Reply #8 on: March 22, 2013, 06:31:04 AM »

Quote
My RFI source is such that I can arrange a null to point
at it if I use a loop

Just an aside, but have you tried to determine
the source of the noise? (what you call RFI)
Is it man made?
(Loose or damaged power line insulator, etc)
Or is it just to generic to tell?



The RFI location story is a long one, I have recently, (the last year), gotten all of the RFI sources down to only two remaining...  We had something like 5 or 6 sources, all very loud, (40 meters was 40 db over S9), and as we have been pealing the onion we are now down to two remaining sources...
Logged

Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
NK7Z
Member

Posts: 756


WWW

Ignore
« Reply #9 on: March 22, 2013, 06:32:44 AM »

You might also look at a half square as an option. This would be equivalent to
having phased verticals for 30m and will give you some gain and a bit of directivity
over a single vertical.

Try this article for starters :

http://rudys.typepad.com/ant/files/antenna_halfsquare_array.pdf

Michael VE3WMB




Hi,

I have a largish lot, some RFI, and a need to operate 30 Meters...  Been thinking of a full sized vertical, (with radial field), but the RFI issue slows me down there...  Also been thinking of a loop...  I have zero experience with loops...  Has anyone used both, and if so how are they for DX?

Thank you Michael,

I may put up the half square, in stead of the Bobtail!!!
Logged

Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
NK7Z
Member

Posts: 756


WWW

Ignore
« Reply #10 on: March 22, 2013, 06:42:11 AM »

If I failed to thank everyone individually, sorry, thank you to all who responded!!
Logged

Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
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