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Author Topic: 40 / 20 vertical  (Read 1238 times)
KR4TH
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Posts: 45




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« on: October 21, 2012, 04:41:09 AM »

Several years ago I used a HF 5 butternut vertical.  Currently using a 40 m dipole (up 30 ft) with a tuner.  I would like to build a more efficient antenna for 40 and 20.  I have read many good reports from those who are using 5/8 wave verticals on 20.  It seems like a 41 foot vertical with an additional 7 foot wire that comes back toward ground (look like an upside down letter J) would radiate as a 5/8 wave and yet tune as a 3/4 wave antenna, about 90 ohms. 

It would appear that the antenna would load on 40 as a 3/8 wave around 100 ohms at the feed point.  The butternut was 26 ft tall and  worked great on 20 as a 3/8 wave and tuned at the base with 1/4 wave of 75 ohm coax for a flat swr reading on 20. 

The base of the vertical would have a stepdown 2:1 torid and fed with 50 ohm coax.

The purpose of the antenna would be a lower angle of take off for 20 and provide a higher feed point impedance on 40 and 20.

Are my assumptions correct?

Jerry
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K3VAT
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Posts: 705




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« Reply #1 on: October 21, 2012, 05:50:21 AM »

...  I would like to build a more efficient antenna for 40 and 20. ... Jerry

One of the keys to effective and successful vertical antenna systems which is spelled out in the ARRL Antenna Handbook and ON4UN's text is two fold:  1) full 1/4 wavelength radiators, and 2) adequate and properly installed radial fields. (a third is having your antenna system away from nearby objects that may substantially interact with it such as close by metal structures.)  A quarter-wave radiator with 'good' radial field is hard to beat and has a number of positive attributes including manageable height on 40M (33') and a feedpoint impedance that may match your 50 ohm coax without matching transformers installed at the feedpoint (a RF Choke is recommended).

IMHO, if you have room use a quarter wave vertical 40M and install an Unadilla 20M trap in it.  The antenna height will be approx 30'.  Try and lay down at least two dozen radials, around 30' each is adequate (more gives minor improvement).  I've used this setup successfully.  This setup is less problematic than your proposal of using 5/8 wave on 20M, and 3/8 wave on 40M, both mechanically and electrically.  In the vast majority of settings, there is no clear-cut superior advantage of the 5/8 wave over the 1/4 wave on the HF bands.  Perhaps someone can do a quick model for you and compare the two designs.

GL, 73, Rich, K3VAT
« Last Edit: October 21, 2012, 06:39:23 AM by K3VAT » Logged
WX7G
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Posts: 5975




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« Reply #2 on: October 21, 2012, 07:42:13 AM »

I modeled your antenna idea on 20 meters in NEC and it doesn't behave as envisioned. Oddly it develops high angle lobes as the wire from the top downward is lengthened toward the target length. The input impedance is as described is 58 -j360 ohms.
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W5DXP
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« Reply #3 on: October 21, 2012, 08:28:40 AM »

Are my assumptions correct?

No, but that may not be important to your satisfaction with the antenna. The difference in length between a 5/8WL vertical and a 3/4WL vertical is the last 1/8WL toward the tip of the antenna. That 1/8WL doesn't contribute much to the radiation pattern because of the low current. It is the current distribution in the rest of the antenna that results in the radiation pattern. The current distribution in a 5/8WL vertical is NOT the same as in a 3/4WL vertical with 1/8WL of the top folded down in an upside down 'J'.

A 3/4WL vertical is known to have a high TOA (take-off-angle). Folding the ends downward (or adding a top hat at the 5/8WL point) doesn't appreciably change that high TOA. However, a high TOA is not all that bad on 40m unless you are looking for an antenna optimized for DX. Lots of hams are satisfied with the good "localized" performance of a 3/4WL vertical and the low resonant feedpoint  resistance. In fact, it is hard to tell the difference between a 3/4WL vertical and a 1/4WL vertical on the air at the TOA of the 1/4WL dipole.

Note: The TOA is defined as the elevation angle of maximum gain.
« Last Edit: October 21, 2012, 08:33:01 AM by W5DXP » 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.
W5DXP
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« Reply #4 on: October 21, 2012, 10:02:10 AM »

However, a high TOA is not all that bad on 40m unless you are looking for an antenna optimized for DX.

Sorry, I was confused about the band on which the antenna is 3/4WL. A 3/4WL vertical is not a good idea on 20m. For DX, it is a little worse than a 1/4WL vertical. A 20m rotatable dipole will beat it by about one S-unit.
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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.
WB6BYU
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Posts: 13149




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« Reply #5 on: October 21, 2012, 10:02:33 AM »

The added wire at the top changes the current distribution on the vertical section of the antenna.

But there are several ways you can make a dual-band antenna for 20 / 40m.  One of my favorites
is to use a quarter wave antenna on 40 with a matching network that handle the 1/2 wave on 20m
at the same time:  this consists of a series LC circuit resonant on 40m that provides the required
L-network inductance on 20m, and a shunt capacitor from the antenna to ground.  I've used this
combination on 20/40 and on 40/80 with good results (though the SWR bandwidth is narrower
on the lower band.)

A longer antenna will have some improvement in signal strength:  there about 1dB of variation
(measured at 15 degrees elevation) on 20m with antenna lengths between 37' and 50', so you
can choose a length that makes it convenient to match on both bands.  The 50' antenna is about
3/4 wave:  at resonance there the resistance on 40m is probably 200 to 250 ohms.  If you used
a 40m quarter wavelength of 93 ohm coax between the antenna and the main feedline it would
transform this down (once you dealt with the reactive component) without affecting the match
on 20m.

In this range the resistance increases with height on 40m while decreasing on 20m.
Somewhere around 43' the resistive values on both bands are about 90 ohms, so once you
arrange to cancel out the reactive components you could use a 3 : 4 turns ratio transformer
at the base to match the resistance on both bands.

All of these impedances depend on your local ground conditions, radiator diameter, etc., but
I don't see any reason that you couldn't make a dual-band antenna by simply providing
matching at the base as needed.  It isn't too difficult to find combinations that will match
on both bands without any switching or manual tuning once properly set up.
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KR4TH
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Posts: 45




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« Reply #6 on: October 21, 2012, 01:21:39 PM »

Thank you for your valuable suggestions and comments

Jerry
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