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Author Topic: Theoretical maximum losses for a vertical  (Read 13304 times)
K1ZJH
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« on: September 23, 2014, 07:35:06 AM »

My friend and I were having a discussion. I had an assumption that at worst resistive ground losses for an inverted L working against a very modest radial system would not exceed 3 dB; based on the current loses due to the ground resistance. Am I looking at this wrong?

I may be able to do three or four radials for my 160 meter inverted L... what is the worst case losses that I can expect?

My needs aren't that great... I can generate a kW, and I would be happy being to achieve DXCC at some point. I am not a serious contester, nor do I expect to beat out any serious 160 meter stations. So, exactly what would I be losing? 

Pete
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KH6AQ
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« Reply #1 on: September 23, 2014, 08:47:17 AM »

It depends on the inverted-L radiation resistance and that depends on the vertical and horizontal dimensions.

Let's say a modest radial system has base-referred loss resistance of 20 ohms. If the inverted-L radiation resistance is 20 ohms the loss is 50% or 3 dB. Figure a good ground system is 5 ohms.

The radiation resistance of a 90 degree 1.8 MHz inverted-L based on the vertical dimension is:

30', 6 ohms
40', 9 ohms
50', 13 ohms
60', 18 ohms
70', 22 ohms

If you install less than eight radials they should not be 1/4 wavelength as that length results in maximum loss. The N6LF radial papers explain this. Instead, they should be 1/8 wavelength.
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G3TXQ
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« Reply #2 on: September 24, 2014, 05:10:17 AM »

This chart may be of some interest: when I installed my 40ft top-loaded 160m vertical, I measured the ground loss resistance as I installed each radial; the radials were typically 30ft to 40ft long:



One word of caution when doing these efficiency calculations - the ground loss resistance for a particular installation of radials varies depending on the antenna; so, for example, my ground loss resistance figures would likely be higher if used under a longer vertical.

Steve G3TXQ
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WB6BYU
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« Reply #3 on: September 24, 2014, 09:02:55 AM »

Now you have some data on both radiation resistance and ground loss resistance.

Assuming that both sets of numbers apply to your specific circumstances, you can calculate the
efficiency as  Rr / (Rr + Rg), where Rr is the radiation resistance and Rg is the ground loss resistance.

But clearly whenever the ground loss exceeds the radiation resistance the antenna will be less than
50% efficient (3dB down).

With 3 radials, G3TXQ's data suggest a ground loss of 13 ohms, which corresponds to an inverted L with
a vertical height of 50'.  Anything shorter than that will have a loss greater than 3dB.
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AA4HA
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« Reply #4 on: September 24, 2014, 10:57:16 AM »

Very informative thread. You guys did a great job in keeping it straightforward for him.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
Free space loss (dB) = 32.4 + 20 × log10d + 20 × log10 f
K1ZJH
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« Reply #5 on: September 24, 2014, 01:01:56 PM »

The vertical portion of my inverted L is at least sixty feet. I suspect I will be able to do a modest radial system to keep losses in the 3 dB range, or maybe a bit better. Thanks for the information--it confirms what I had hoped for.

Pete
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RFRY
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« Reply #6 on: September 25, 2014, 01:52:25 AM »

The ESR of a set of buried radials also depends on the conductivity and permittivity of the earth in which they are buried (see below).

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W0BTU
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« Reply #7 on: September 25, 2014, 06:03:13 PM »

Hi Rich,

My 160 meter inverted-L (http://www.w0btu.com/160_meters.html#inv-l_antenna) has two λ/4 resonant radials, 10' high and 180 degrees apart. Assuming the currents are equal in each one (which they probably aren't), I wonder what the loss is and what it might be with four or six of them?

An EZNEC file is at http://www.w0btu.com/files/antenna/160m_inv-L/ but I don't have the NEC4 engine.

Whatever the case, I'm pretty confident that we can use far fewer radials if we elevate them vs. laying them on the earth (or in a shallow trench).

In any case, even with 120 radials --at any height-- the grounds losses farther away from the antenna system don't go away.
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RFRY
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« Reply #8 on: September 26, 2014, 05:36:54 AM »

Hi, Mike -

I downloaded your EZNEC model of an inverted L and added another pair of elevated radials using the same XY elevations as the first pair, but along the Y axis (90o from the first pair).

Here is what a NEC4.2 far-field analysis shows for the original and modified models over 5 mS/m, d.c. 13 earth.

Using 4 elevated radials instead of 2 brought the peak elevation gain up by about 1.7 dB.

Quote
Whatever the case, I'm pretty confident that we can use far fewer radials if we elevate them vs. laying them on the earth (or in a shallow trench).

Agreed.



« Last Edit: September 26, 2014, 05:41:01 AM by RFRY » Logged
KH6AQ
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« Reply #9 on: September 26, 2014, 06:04:54 AM »

Hi Rich,

My 160 meter inverted-L (http://www.w0btu.com/160_meters.html#inv-l_antenna) has two λ/4 resonant radials, 10' high and 180 degrees apart. Assuming the currents are equal in each one (which they probably aren't), I wonder what the loss is and what it might be with four or six of them?

An EZNEC file is at http://www.w0btu.com/files/antenna/160m_inv-L/ but I don't have the NEC4 engine.

Whatever the case, I'm pretty confident that we can use far fewer radials if we elevate them vs. laying them on the earth (or in a shallow trench).

In any case, even with 120 radials --at any height-- the grounds losses farther away from the antenna system don't go away.

Looking at the link, you have the information and the knowledge to indirectly determine the radiation efficiency. Here's how I do it: Using NEC include the L-network and a series resistor is placed right above the feedpoint. The resistor value is adjusted until the simulated VSWR bandwidth matches the measured VSWR bandwidth. Calculate the efficiency.
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KF4CQR
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Posts: 36




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« Reply #10 on: September 27, 2014, 10:06:15 AM »

Good thread!

Not trying to hijack this, only expand with a question.

Question:   What would be the losses on 80 meters with the same antenna dimensions?  I other words, what losses can I expect with the same antenna lengths/heights as at or near 1/2 wave length on 80 meters?

My issue is I love working local on 80 meters... but I live in a restricted area and a small wire is much more stealthy than coax or balanced line.


Thanks


Jerry
KF4CQR
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W0BTU
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« Reply #11 on: September 27, 2014, 11:32:24 AM »

... Using 4 elevated radials instead of 2 brought the peak elevation gain up by about 1.7 dB. ...

Thanks, Rich! And I see that in some directions, the gain is 3 dB higher.
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W0BTU
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« Reply #12 on: September 27, 2014, 12:07:54 PM »

Hi Dave,

Looking at the link, you have the information and the knowledge to indirectly determine the radiation efficiency.

I appreciate your vote of confidence. :-)  When I saw Richard Fry's model, I was hoping he'd take my subtle hint, and I'm grateful that he did. I'm certain he (and you) has more experience in that field than I do. If I can duplicate his results here in EZNEC+ 5.0, then I can be more or less certain that adding more elevated radials to my model will produce accurate results.

Quote
Here's how I do it: Using NEC include the L-network and a series resistor is placed right above the feedpoint. The resistor value is adjusted until the simulated VSWR bandwidth matches the measured VSWR bandwidth. Calculate the efficiency.

Thank you for that! I've never tried to add anything but wires and sources inside EZNEC, but I have a printout of the EZNEC+ 5.0 manual right here in a ring binder. Now all I need to is find the time and energy.  Smiley
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RFRY
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« Reply #13 on: September 27, 2014, 12:48:06 PM »

This link leads to a post by William Culpepper, a broadcast consulting engineer, showing that his measurements of the efficiency of monopole systems using only four 1/4-wave elevated radials was very nearly the same as when using a set of 120 x 1/4-wave buried radials (other things equal).

http://lists.contesting.com/_topband/2007-11/msg00248.html
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W0BTU
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« Reply #14 on: September 27, 2014, 01:08:20 PM »


I don't recall ever reading that. Good info!

I just added this info to www.w0btu.com.
« Last Edit: September 27, 2014, 01:47:21 PM by W0BTU » Logged

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