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Author Topic: Multi-band (?) vertical antenna  (Read 2744 times)
KB3VQQ
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Posts: 11




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« on: July 20, 2012, 07:04:17 AM »

Hi All,  I have a question for you antenna elmers out there...

In setting up a multi-band vertical, I know that as a general rule, you should choose a length that is non-resonant on all the bands you want to use and specifically avoid anything that would be a multiple of 1/2 wavelength...

My question is how far away from a 1/2 wavelength do you have to be to still be able to tune it up?  (I'm using an MFJ-993B on the 150W setting and a "barefoot" Yaesu 450 at 100W PEP max.)

Also, I read somewhere that even though the recommendation is to set the length for 1/4 wavelength on the lowest band of operation, you can get by at 80% of that length.

For example, 234/14.070 would give a 1/4 wavelength of 16.6 feet to be resonant for 20m PSK31 operation...  But 16.6 feet is also 1/2 wavelength on 10m.  If I go with 80% of the 20m wavelength, that would be 13.3 feet so if I settle on let's say a 14ft vertical element, it would be 84% of the 1/2 wavelength on 10m (14 ft versus 16.6ft)...

Would that work or am I getting my hopes up too high?  My main goal is to set up a vertical for 20m digital, but I'd really like to be able to use it the higher bands as well...  This is a homebrew copper pipe project and while buying a trapped vertical would be nice, it's not in the cards right now.  I was also trying to keep the construction simple and strong to hold up to Erie PA winters!

Any advice is appreciated!

Thanks!
Josh
KB3VQQ
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AA4PB
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Posts: 12700




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« Reply #1 on: July 20, 2012, 07:46:49 AM »

My first concern is the use of the MFJ933B, which is a balanced tuner intended to be located at the base of a loop antenna. I don't know how it would perform when attempting to tune an end-fed antenna like a vertical - especially if it were connected to the vertical via coax cable.

Given a "typical" tuner the goal is to cause the antenna feedpoint impedance to be something reasonable, which is why you don't want to make it a multiple of 1/2 wavelength (which would present a very high feed impedance). The feed impedance will also depend on the number of radials and (to some degree) the type of soil. On the other hand, if the tuner is located at the base of the antenna and has the required tuning range then a 1/2 wave end-fed antenna has the benefit of less current at the feed point and therefore less loss in the radial system.

The other consideration is that the radiation take-off angle changes with the antenna length. High angles are better for close-in stations while low angles are better for greater distances. For example, 20M is a good DX band so you probably wouldn't want to have a take-off angle of 50 degrees on that band.

The best advise is probably to use one of the antenna modeling programs to model different lengths to determine the feed impedance and take-off angle on the bands of interest.

You might also want to consider a mechanical arrangement where you can change the physical length of the antenna to provide the best performance on each band. This could be, for example, a vertical wire supported by a fiberglass pole.

Also remember that loss makes the SWR look better but it does that by converting RF into heat rather than radiating it. There are antennas that include resistors in order to create loss and flatten the SWR in order to make an all-band, no tuner required antenna.
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K1WJ
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Posts: 451




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« Reply #2 on: July 20, 2012, 08:11:31 AM »

Maybe something like the MFJ-1976 telescoping vertical would be ideal - adjust length of element for 6,10,12,15,17,20m
A Full 1/4 wave for each band. This antenna stays where adjusted & works great ground mounted with has many ground radials has you can fit.
Also works very well elevated about 6ft with 2 1/4w radials also held up on ends 6ft - great for a single band - I prefer 17m or 20m this configuation. Catch you on the airwaves with mine. Grin

http://www.mfjenterprises.com/Product.php?productid=MFJ-1979

73 K1WJ David Cool
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WB6BYU
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Posts: 13040




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« Reply #3 on: July 20, 2012, 08:34:26 AM »

Quote from: KB3VQQ

In setting up a multi-band vertical, I know that as a general rule, you should choose a length that is non-resonant on all the bands you want to use and specifically avoid anything that would be a multiple of 1/2 wavelength...



Actually I prefer to use 1/2 wavelength verticals when I can.  They work well, and
I've never had problems matching them.

The limitation is in the automatic antenna tuners that use relay-switched coils and capacitors,
because the half wave antennas have a high feedpoint impedance, hence high voltages that
can arc the relay contacts.

In fact, there is a simple matching network that uses two capacitors and an inductor that
will match such an antenna simultaneously on two bands, such as a 80m quarter wave on
40m, or a 20m quarter wave on 10m.  I've used this on both 80/40m and 40/20m over the
years.  Narrows the SWR bandwidth somewhat on the lower band, but otherwise works fine.
Put a capacitor and coil in series between the coax inner conductor and the antenna, such
that the combination is series resonant on the low band, and on the high band it provides
the proper inductance to match the high impedance in conjunction with a small capacitor from
the antenna to ground.  Requires a bit of setting up, but then the antenna is matched on
both bands without an additional tuner, and probably on 12/15/17m with a tuner.


Quote

My question is how far away from a 1/2 wavelength do you have to be to still be able to tune it up? 



That depends on the specifications of your tuner, the radiator diameter, ground system losses, etc.

But as an example, S9 sells 29' verticals for use on 40m and 20m, so 14 1/2' should work for 20/10m.



Quote

Also, I read somewhere that even though the recommendation is to set the length for 1/4 wavelength on the lowest band of operation, you can get by at 80% of that length.



There are a lot of things you can "get by" with, often at the expense of efficiency.  Depending
on the feed and loading methods, you can go down to half size or smaller, though somewhere
around 2/3 is probably a better lower limit.  But making the whip 80% of a quarter wave on the
lowest band will certainly work - a friend has the 29' S9 vertical I mentioned previously and
does quite well on 40m (though he has added a bit of base loading to it so he doesn't need
a tuner in the shack.)

The other thing to consider is making the antenna LONGER than 1/4 wave on the lowest band,
as that improves the efficiency somewhat.  Conventional wisdom is that the antenna shouldn't
be longer than 5/8 wave on the highest band, though performance really doesn't degrade much
until you get over 3/4 wave in the practical world.  Certainly if you want to work 30m or 40m,
this would be preferred.  Basically it would be the equivalent of the common 43' vertical cut in
half, though 20' should work if that is a convenient radiator length.  (The issue would be to
avoid 1/2 wave on 15m or adjacent bands.)  I remember an article in QST many years ago
using a 24' vertical on all bands 10m through 80m with a manually-switched matching network
in a box at the base.  (Manual switching avoids the problems with relays in autotuners.)
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W5DXP
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« Reply #4 on: July 20, 2012, 08:55:46 AM »

For 40m-10m operation, it's hard to beat a 22' vertical with the feedpoint 22' in the air and four 22' radials sloping down at a 45 degree angle. Feed it with an autotuner at the feedpoint and enjoy a 4+ dBi gain on 10m with a TOA of 11 deg. The gain on 40m is ~0.5 dBi at 22 deg, better than a 1/4WL ground mounted vertical.
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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.
K3VAT
Member

Posts: 701




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« Reply #5 on: July 20, 2012, 09:39:44 AM »

For 40m-10m operation, it's hard to beat a 22' vertical with the feedpoint 22' in the air and four 22' radials sloping down at a 45 degree angle. Feed it with an autotuner at the feedpoint and enjoy a 4+ dBi gain on 10m with a TOA of 11 deg. The gain on 40m is ~0.5 dBi at 22 deg, better than a 1/4WL ground mounted vertical.

Hi Cecil,  Thanks for this post.  Off hand, do you know the feedpoint impedance of this antenna for 7.1mhz?  Thanks! 73, Rich, K3VAT
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KF7CG
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Posts: 817




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« Reply #6 on: July 20, 2012, 10:32:04 AM »

Try the MFJ-1798 for low radiation angle, no radials, vertical. It works quite well on all bands 80 through 2 with reduced bandwidth on 80, 40, and 30.

Its main draw back is that it is somewhat fragile mechanically. It uses top feed and a large top mounted couterpoise to achieve its no radial performance. Works well if the local winds stay less than 40-50 mph.

KF7CG
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W5DXP
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Posts: 3546


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« Reply #7 on: July 20, 2012, 11:45:01 AM »

Off hand, do you know the feedpoint impedance of this antenna for 7.1mhz?

EZNEC sez 24 - J 235 ohms (doesn't include ~10? ohms of ground losses). A +j235 ohm base loading coil would probably give an SWR around 1.5:1. My SG-230 did a good job of matching it on all bands 40m-10m.
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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.
K3VAT
Member

Posts: 701




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« Reply #8 on: July 20, 2012, 12:00:31 PM »

Off hand, do you know the feedpoint impedance of this antenna for 7.1mhz?

EZNEC sez 24 - J 235 ohms (doesn't include ~10? ohms of ground losses). A +j235 ohm base loading coil would probably give an SWR around 1.5:1. My SG-230 did a good job of matching it on all bands 40m-10m.

Cecil,
Thanks!  Very decent Z numbers and easy to match.  You're correct, this would be a very good antenna for someone with small lot and the ability to erect a 22' wooden/fiberglass post and install 4 elevated radials.  Gain numbers are decent too!  Nice setup for 10M through 40M and beats those 43 footers on several bands for DX. 73, Rich, K3VAT
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KB3VQQ
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Posts: 11




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« Reply #9 on: July 20, 2012, 12:37:55 PM »

Thanks for the responses everyone! 

Cecil:  Can you tell us what the numbers would be for the 22' configuration on 15m?  (looking at 21.070)  I ask because that would be the one band where 22' is 1/2 wavelength...

Thanks!
Josh
KB3VQQ
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W5DXP
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Posts: 3546


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« Reply #10 on: July 20, 2012, 04:46:21 PM »

Cecil:  Can you tell us what the numbers would be for the 22' configuration on 15m?  (looking at 21.070)  I ask because that would be the one band where 22' is 1/2 wavelength...

Guess I should feature that antenna on my web page. Here are the feedpoint impedances, gain, and TOA figures for all bands, 40m-10m, for a 22' vertical, elevated to 22', with 22' radials sloping down at 45 deg.

7.15 MHz, 24-j233 ohms, 0.5 dBi @ 22 deg TOA
10.125 MHz, 46-j31 ohms, 0.9 dBi @ 19 deg TOA
14.2 MHz, 128+j233 ohms, 1.5 dBi @ 16 deg TOA
18.14 MHz, 556+j584 ohms, 1.9 dBi @ 14 deg TOA
21.3 MHz, 1315-j372 ohms, 2.5 dBi @ 12 deg TOA
24.95 MHz, 373-j560 ohms, 3.4 dBi @ 13 deg TOA
28.4 MHz, 85-j290 ohms, 4.2 dBi @ 11 deg TOA

My SG-230 didn't have any trouble matching it on 15m. My only problem with it was the receive noise level caused by very old power lines at my last QTH.
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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.
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