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1/4 wave ground plane SWR and POWER, antenna theory

Joined
Jul 13, 2014
Messages
57
Location
Central Texas
#1
Today i built my first antenna. A simple 1/4 wave ground plane with copper wire and a SO239 connecter. What i found very interesting was when i was complete and i started to check power and swr, that i was able to tune the SWR and in turn, adjust power out, while changing the angle of the radial elements.
I first started with them almost fully horizontal. I first tried at 5 watts just to check everything out. I was only able to get 4 watts out and an SWR of 3.0:1. After adjusting them closer to 45 degrees down, I was able to bring the power up to 5 watts with an SWR of 1.5:1, so i kept going. Adjusting them to almost 90 degrees i was able to get a full 5 watts with so little noticeable bump on the SWR meter the it was lower than 1.1:1.
I found this very interesting and tried it on 2 other frequencies that were a ways apart, and again it started at lower power with a high swr, and after adjusting, i was able to TX full rated power with a SWR of 1.1 or less.
All of this was done without any trimming of the radials, which i expected that i would need to do.

Can someone explain how i was able to basically "tune" this antenna by adjusting the radial elements instead of adjusting the length?

Also, im guessing that by adjusting my radial elements lower than 45 degrees that im actually effecting the radiating pattern. So am i better to actually trim the elements with them at 45 degrees to adjust swr and power, so that my radiation pattern is ideal, or adjust the angle to have 1 antenna that can be tuned to be resonant on different frequencies based off of the angel of the radiating elements?

Im wanting to get into antenna theory, so yes im very new to this, but someone please educate me.
 
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jwt873

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Dec 1, 2015
Messages
843
Location
Woodlands, MB
#2
A ground plane antenna has an impedance of around 30 ohms.. Your radio needs to see 50 ohms.

There is capacitance and inductance between the radials and the driven element. Drooping the radials changes values of the inductance and capacitance which in turn brings the impedance up to the 50 ohms your transmitter wants to see.
 
Joined
May 28, 2009
Messages
2,604
#3
19dsniper - so you asked for antenna theory. Sure enough, on paper, a ground plane antenna with horizontal radials is roughly 30 ohms impedance.

As you move the radials downward, the impedance rises. If you move them down far enough, so much so that it resembles a vertical dipole, the impedance should be near 50 ohms.

(We aren't talking about trimming things yet)

The point I'm making is that keep in back of your mind the fact that when dealing with a ground plane at 30ohms, with an instrument expecting 50ohms:

Minimum swr will NOT mean you are "resonant". They will differ slightly.

Those who tune for the lowest swr with the horizontal ground plane, may actually make their antenna LESS efficient.

The basic rule of thumb work-around for this is that when tuning a horizontal ground plane antenna, you trim elements so you are slightly off, say a 1.5:1 reading, and not 1:1.

There are many more things to consider, but tuning for minimum swr is not always the correct answer.
 
Joined
Dec 25, 2008
Messages
3,147
Location
New Zealand
#4
What hertzian says +1.

The resonant frequency versus low SWR has always been a topic of conversation - as you have found, you can get the SWR down low by bending the radials down, but because you get the lowest SWR with the radials bent right down, then it looks like your radiating element, the vertical bit, is not the correct length for the frequency. Where did you have your SWR meter installed? Down the end of a long bit of coax cable is not the right place, it need to be close to the antenna because the coax cable becomes part of a non-resonant antenna and modifies the SWR reading - it makes it look better than it is. Because you think you were pumping more power into it, you were probably dissipating power in the coax. In a typical ground plane antenna the radials are bent down at 45 degrees and you trim the length of the radiator for lowest SWR or better still best field strength at a distance. So calculate the length of the radiator for your frequency then add a few inches. Then start chopping off the excess in half-inch increments - not while power is applied! Even a few watts will give a nasty RF burn at VHF!. The posh way is to have a telescopic radiator, find the right length, then make a permanent radiator to that length. Get well away from the antenna when measuring and have it fitted in it's final location - any metalwork nearby will modify your readings. You must have 50ohm cable for a ground plane - so RG6 is out - 75 ohm is fine for dipoles but if your transmitter has a 50ohm output you'll have to think again!. Don't pay any attention to those who say that the coax must be in half wavelength multiples to get the correct reading!
 
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Joined
Jul 13, 2014
Messages
57
Location
Central Texas
#5
Thank you for the additional information. I honestly haven’t messed with it since then. I was using the included 50 ohm cable that comes with the analyzer to run from the radio to the analyzer (maybe 1 foot long) then used a 3 foot jumper of LMR400 flex cable from the SWR meter to the antenna. I am unable to test it yet in its final location as I threw away the idea of a shorter antenna base and I’m going to go ahead and put up 30-40 feet of Rohn 25. I will however, research this more and continue to play with more antenna builds as I find it not only very interesting but also a good skill to have! Thanks for the additional information!

ETA: radiating element was trimmed to 19.25”. I will have to go back and measure what I have the horizontal elements cut at. I think it’s the same. I was just following along with a YouTube video I watched.
 
Joined
Feb 6, 2007
Messages
5,934
Location
175 DME, HEC 358° Radial
#6
This brought back a distant memory. I was about 12 when I got my first scanner, and I wanted to build a ground plane. My dad offered to help, and I ended up learning how to calculate the length of a coax stub to put on the antenna to match that 30 ohm gp to 50 ohm coax.

BTW, there are commercial ground plane antennas that bend the radials down for impedance matching. Sounds like you rediscovered a commercially viable technique quite by accident.
 

W5lz

Member
Joined
Feb 28, 2019
Messages
189
#8
There are always two parts to an antenna. The second half may be several, or a number of severals (howzat for confusing?). The feed line, dirt, another element or whatever, it's always there somewhere.
The way to tune an antenna is first for resonance e, then for impedance matching. It's also the hardest way, but still the best. (Resonance means most efficeint ... not responsible for spelling.)
 

KB4MSZ

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Mar 12, 2018
Messages
300
Location
Tampa, Florida
#9
The way to tune an antenna is first for resonance e, then for impedance matching. It's also the hardest way, but still the best.
Exactly. I spent yesterday tuning my mobile mounted stainless steel 17' telescopic whip to each ham band, writing down how many sections where collapsed and how much remaining of the next above section. Using an analyzer, the frequency was adjusted for the center of the phone bands (don't plan to operate CW in my vehicle), with the reactance reading zero. The match isn't always 50 ohms but is pretty close. Some bands are dead on. I can just pull the antenna out to the band I want, the list is stored in a PDF within my phone.

Now if we just had some decent conditions.
 

Ubbe

Member
Joined
Sep 8, 2006
Messages
3,012
Location
Stockholm, Sweden
#10
Amateur radios can be fine impedance wise. But scanners are totally off the scale.
I've made an analyse of my MD380 UHF at 422Mhz, 446MHz and 468MHz and that looks good but could use a bit of adjustment in the lower band.

/Ubbe




 

W5lz

Member
Joined
Feb 28, 2019
Messages
189
#11
Scanner inputs are typically somewhere around 50 ohms too. And with an antenna that's close to 50 ohms it wouldn't be much "bad" reception. Receivers just are not as picky as transmitters. Actually, the antenna's impedance isn't going to make as much difference as the antenna's resonancy. Getting that antenna higher will make a huge difference rather than it's input impedance will. With the typical scanners freq range, getting resonance in more that one spot (or a harmonic of that spot) is almost impossible. So, manufacturers don't worry about, and you shouldn't either...
 

Ubbe

Member
Joined
Sep 8, 2006
Messages
3,012
Location
Stockholm, Sweden
#12
Scanner inputs are typically somewhere around 50 ohms too.
The BCD536 have a 20 ohm impedance at 390MHz and 170 ohm at 445MHz. Most scanners have the same typical variations in impedance over a frequency range. The $100 MD380 have more or less 50 ohm impedance over its whole frequency range.

/Ubbe
 
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