1/2 ground plane for 11 and 10 meters

[VK3FTIM]

Gday all
During the week i've had some spare wire and time on my hands lately

So i chose to built a 10 Meter Vertical ground plane antenna and it works perfectly

i got 5.24 meters of wire or 17.19 feet for you yanks

i did 5.24 divided by 2 and got 2.62 meters (or 8.59 Feet) for the hot element
now i had a spare 2.62 meters (or 8.59 Feet) of wire for the ground plane i cut each ground plane for 1.31 meters (or 4.29 feet)

No balun needed for this antenna design as it matches the 50 ohm coax

soldered it all up together and hoisted it up

it is a very broadband antenna at 25.0MHZ the VSWR is 1.5 at 27.355 it is 1.3 at 28.350 it's 1.2 and at 29.5 it's 1.4

i'll post the specs and pictures later tonight all

[G3TXQ]

Those are very surprising VSWR results for the antenna that you describe. The low VSWR over such a wide frequency range would indicate significant losses in the system. How long was your 50 ohm coax feedline, and what type of cable was it?

Steve G3TXQ

[VK3FTIM]

Gday Steve

it was RG58 c/u and it was 1/2 wave length long 5.24 Meters straight into the Tuner in bypass mode and then into the VSWR Meter then into my Icom IC718

it was hoisted approx 7 meters above ground and the radials were 120 degrees apart facing the east - west

The coax it prehaps acting as a "matching stub" if anyone would like to correct me on that

[G3TXQ]

A few comments:

1. 5.24m of RG58CU is not a half-wavelength at 28MHz - you're forgetting to account for the velocity factor of the cable. It's actually about 0.74 wavelength.

2. The 5.24m of coax is not really a "matching stub" - it does nothing to alter the VSWR seen by the transmitter.

3. Your radials are half the length they should be for resonance, which means that they present quite a high impedance at the feedpoint. If you are measuring a reasonable VSWR, chances are that the "return currents" are not flowing in the radials but along the outer surface of the coax braid. Try changing the length of the coax by about 1.7m and see if the VSWR changes; it shouldn't, if the antenna is working as a proper groundplane!

73,
Steve G3TXQ

[G3TXQ]

OK - just modelled your antenna using EZNEC; I assumed that your 5.24m of feedline was well earthed at the shack end. I get a very good match to 50 ohms!

The reason is that the outer surface of your coax is acting as a half-wavelength "radial" which is earthed at its end furthest from the feedpoint; that means the surface of the coax provides a very low impedance for the "return current". In the model, almost 10 times as much current is flowing in the coax braid as in the short radials. Note that the 5.24m IS a half-wavelength in this case because the velocity factor correction doesn't apply.

There's a couple of interesting experiments you could do:

1. Change the length of the coax by 2.5m and see what difference it makes to the VSWR - I expect there will be a large change.

2. Keep the coax at 5.24m and remove both of the short radials. What happens to the VSWR? I expect the change will be small.

73,
Steve G3TXQ

[AI3V]

Gday Steve
The coax it prehaps acting as a "matching stub" if anyone would like to correct me on that

ANY feedline, Connected to ANY antenna, or load, Where the characteristic impedance of the cable, and the conjugate impedance of the load are not equal IS A MATCHING SECTION.

EDIT: For Steves post below.

Wether or not the line improves. or worsens the VSWR depends on many factors.

/EDIT

Unless VSWR = 1:1 there WILL BE a impedance transformation in the feedline.

A 1/2 wave (electrical length) feedline has the unique property that the impedance transformation has gone "once around the (Smith chart) VSWR circle", and will repeat the feedpoint impedance at the other end, minus feedline losses.

If you were able to "slide" your VSWR meter along the 1/2 wave section, with the line impedance,and the load impedance not equal (VSWR>:1),you would see the indicated reading vary in a sinusoidal manner , repeating every 1/2 wave, with the absolute VSWR slowly decreasing as you move farther away from the load due to line losses.

Rege

[G3TXQ]

ANY feedline, Connected to ANY antenna, or load, Where the characteristic impedance of the cable, and the conjugate impedance of the load are not equal IS A MATCHING SECTION.

If that is true, what length of 50 ohm coax matching section do I need to produce a good 50 ohm match with a load of 100 ohms?

He's using 50 ohm feedline and he's looking for a 50 ohm match. No amount of cable can provide a match in that situation ( other than a very lossy length).

A 1/2 wave (electrical length) feedline has the unique property that the impedance transformation has gone "once around the (Smith chart) VSWR circle", and will repeat the feedpoint impedance at the other end, minus feedline losses.

If you were able to "slide" your VSWR meter along the 1/2 wave section, with the line impedance,and the load impedance not equal (VSWR>:1),you would see the indicated reading vary in a sinusoidal manner , repeating every 1/2 wave, with the absolute VSWR slowly decreasing as you move farther away from the load due to line losses.

Indeed, but he doesn't have a 1/2 wave section - he has a 0.73 wavelength section.

Regards,
Steve G3TXQ

[AI3V]

Steve,

You must understand that even tho a feedline acts as a matching section, There is no guarantee that a particular impedance feedline section will be the solution to a particular problem.

May I recommend the book "Antenna Impedance Matching" for a detailed discussion of which type of mismatch can be solved with which types of matching device?

The book will show how to plot your antenna on the Smith chart, and then shows which section of the chart can be matched by which scheme. Then it shows how to realize the solution with real-world components.

Rege

[G3TXQ]

Steve,

You must understand that even tho a feedline acts as a matching section, There is no guarantee that a particular impedance feedline section will be the solution to a particular problem.

May I recommend the book "Antenna Impedance Matching" for a detailed discussion of which type of mismatch can be solved with which types of matching device?

The book will show how to plot your antenna on the Smith chart, and then shows which section of the chart can be matched by which scheme. Then it shows how to realize the solution with real-world components.

Rege

Rege,

In my professional career with MoD and BBC I did more matching using the Smith chart than I care to remember - I really don't think I need to refer to the text books again.

My point was specific to the situation that VK3FTIM is describing. He asked if his 50 ohm series coax was acting as a matching stub. The clear answer is "no, it is not". If the required impedance is 50 ohms, series 50 ohm feedline cannot act as a matching stub. Take a look at a Smith Chart and you'll see why

Steve G3TXQ

[G3TXQ]

VK3FTIM,

Thought you might be interested to see the current distribution on your antenna:

http://www.karinya.net/g3txq/temp/groundplane.png

Wire 1 is the radiating element; wires 2 & 3 are your half-length radials; and wire 4 represents the outer surface of the coax braid which is earthed at its bottom (shack) end.

Tha mauve lines represent the current distribution. As you can see, there's very little current flowing in the radials compared to the main vertical radiator or the coax braid. In other words the coax braid has become an important part of the radiating structure; that's usually something we try to avoid!

What's interesting is that the particular length of coax you chose/needed happened to be the right length to produce a good 50 ohm match. Now, while the luck's with you, how about predicting next week's lottery numbers

Edit: just for completeness, here's what happens if you lengthen the coax by 2.5m:
http://www.karinya.net/g3txq/temp/groundplane2.png
The coax becomes close to 3/4 wavelength long, the impedance it presents at the feedpoint is very high, and therefore much more current flows into the short radials. In turn, because those radials are way short of 1/4 wave resonance, the overall feedpoint impedance becomes highly capacitive and the VSWR rockets!

73,
Steve G3TXQ