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Author Topic: Question on inverted L  (Read 832 times)
N8TI
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Posts: 115




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« on: February 12, 2012, 10:07:11 AM »

Hello everyone. I have a 50 foot tower. I have my inverted L go up alongside the tower about 1 foot away then out 70 feet horizontal, sloping to 20 feet off the ground.

I have three radials. Each of them lay on the ground for about 15 feet then are raised six feet off the ground for the rest of their horizontal travel. (On top of plastic fences.)

My question is, whether the portion of the radials on the ground makes a substantial difference in the antenna's transmit  performance? The antenna performs well, but on the higher bands, you can hear the difference when compared to the quad I have on the tower when it is pointed at the station. However, the difference is only about 1, maybe 2, S units, which I would hope for anyway. So, receivewise, the inverted L seems to work OK. The inverted L works and loads well on all bands.

Thanks.

Joe, N8TI




   
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K0ZN
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Posts: 1548




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« Reply #1 on: February 12, 2012, 08:08:42 PM »


 If you only have 3 radials on the ground, in theory, you are probably losing nearly half of your transmitter power to ground losses in that system. i.e. maybe
2 to 3 db.  The losses with only a few radials is very high; not much different than NO radials! You probably need a minimum of about 24 to 30 radials to bring ground system losses down to "acceptable". If you want to eliminate "most" of your ground losses, you would need about 60 to 70 radials.  Now, all that said, even if you lose 40% of your power in ground losses, the antenna will radiate and you can/will get decent signal reports, especially if you are running high power. To wit: if you put a KW into the antenna and lose 40% in ground losses, you still have a 600 watt ERP, which will give a very significant signal. If you are/were running QRP, that would be an ugly loss..... it is kind of relative, so to speak.

There have been a number of studies and modeling done of the number of radials vs. ground loss over the years and they all pretty much get in the same ball park, indicating that only a hand full of radials yields a lot of loss. Remember that plain old dirt is NOT a good conductor of RF energy....that is the point of the radials....to reduce ground loss. The energy is spread equally around that type of antenna, so only a few wires really doesn't provide much in the way of reduced losses. Sorry to be the bearer of
bad news....but it is just Ohm's law!

73,  K0ZN
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NW2K
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Posts: 13




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« Reply #2 on: February 13, 2012, 05:17:49 AM »

On a related matter...with the vertical wire in close proximity to the 50 foot tower, there will likely be significant interaction on 80M and the higher bands.
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KB4QAA
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Posts: 2371




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« Reply #3 on: February 13, 2012, 07:29:13 PM »

No portion of the radial is more important than another, but the flux density is greater the closer to the antenna.  Ideally you would make your radials equal or just slightly longer than your antenna, layed out in a fan shape underneath and to the sides. 

Comparing a Quad to your inverted L is like comparing a Porsche to a Volkswagen.

One reason why your inverted L performance drops off at higher frequencies is the increasing number of lobes at higher take off angles as the antenna length approaches or exceeds the wavelength.  Same thing happens with dipoles used on multiple bands.
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M6GOM
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Posts: 904




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« Reply #4 on: February 14, 2012, 06:52:31 AM »

On frequencies where the length of the inverted L is a half wavelength or longer, ground radials make no difference.

As you get over 1 wavelength, you get all kinds of patterns developing with lobes and nulls which increase in number, depth/gain and narrow in width as the frequencies go higher. Some nulls can be as much as -40dB.
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K0IZ
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Posts: 737




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« Reply #5 on: February 14, 2012, 07:01:31 AM »

Perhaps you could change the feed point on the bottom of the vertical section, to be up in air about 10 feet or so.  Then move your radials to that point (would be 10ft at tower, 6ft at fence).  Three elevated radials should work well, much better than radials on ground (or partially on ground).
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