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Author Topic: Designing extremely short vertical dipole  (Read 7472 times)
JAHAM2BE
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« on: April 15, 2013, 02:31:37 AM »

I got 4nec2 up and running again on my re-installed computer, and I've got a couple of questions on a short vertical dipole design I was playing with (mostly using information and sample antenna designs from N3OX posted here on eham). I'd like to know if things sound reasonable or if I'm misinterpreting the simulator results. The NEC2 code (created with 4nec2) is appended below.

The basic design parameters are:
- 7 MHz operation
- 2m long dipole radiator (each leg 1m), 10cm diameter (will be fabricated by hand by rolling long copper flashing into a wide tube)
- A capacity hat, at each end, consisting of two diametrically opposed copper rods, each 1m long and 1cm diameter
- Loading coils for each arm located at the feedpoint, each 9.6 uH with resistive loss 0.875 ohms (Q=483 at 7 MHz)
- A shunt coil for impedance matching (value not yet determined)
- Vertical orientation, height of bottom-most element 0.5m above city/industrial ground.

Simulating this in 4nec2 I'm getting gain of -6.96 dB at 55 degrees (where 0 degrees is straight up).
http://www.freeimagehosting.net/newuploads/zlwdh.png

Some questions:

1. Does the above gain sound about right?

2. Does the above gain figure mean that at 55 degrees, about 20% of the input power will get radiated? So if I'm running a 5W rig, I would effectively have 1W radiated at 55 degrees?

3. Using 4nec2's optimizer, I can't seem to get a proper value of the shunt coil (across the feedpoint) to match to 50 ohms. Currently the best possible SWR is 9.75, with a shunt coil of 0.59 uH. Is it possible to match this antenna to 50 ohms using a shunt coil in this fashion?

4. Given the low capacity of the simple capacity hats, the required loading inductance is large. To reduce the loading inductance (and inductor losses) I decided to place the coils right at the feedpoint. For this particular antenna, does this seem like a reasonable decision?

If the gain figures are about right, I'm thinking it might be possible to make a portable version of this antenna (four 1m thin rods for the capacity hats, two 1m thick tubes for the radiator) for portable operation with a QRP rig. At home (concrete building and balcony  Angry) the dipole could be tried in various vertical, horizontal, and sloping positions. I figure if at least about 1 watt ERP manages to get out, it could be a useful antenna, about on par with a 40m magnetic loop of the same size but possibly easier to construct.

Any comments or advice appreciated.

NEC2 code follows:

Code:
CE
SY L=9.611e-6
SY shuntL=5.897e-7
GW 1 9 0 0 0.5 0 0 2.5 0.1
GW 2 3 0 0 0.5 0 -1 0.5 0.01
GW 5 3 0 0 0.5 0 1 0.5 0.01
GW 14 3 0 0 2.5 0 -1 2.5 0.01
GW 17 3 0 0 2.5 0 1 2.5 0.01
GE -1
LD 5 1 0 0 58000000
LD 5 2 0 0 58000000
LD 5 5 0 0 58000000
LD 5 14 0 0 58000000
LD 0 1 4 4 0.875 L
LD 0 1 6 6 0.875 L
LD 0 1 5 5 0.06 shuntL
GN 2 0 0 0 3 0.0001
EK
EX 6 1 5 0 1 0 0 0
FR 0 0 0 0 7 0
EN
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G8HQP
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« Reply #1 on: April 15, 2013, 04:11:47 AM »

A short dipole with uniform current flow has (according to Krauss) a radiation resistance of 790(L/lambda)^2. 2m at 7Mhz gives 1.72 ohms. You have 1.75 ohms of loading coil resistance, plus a little more copper loss in the elements themselves. Also, your current won't be uniform - an unloaded short dipole has only one quarter the radiation resistance. Let's take a wild guess that your radiation resistance is 1.5 ohms and your loss resistance is 2.0 ohms. You will then be (1.5/3.5)^2 or -7.36dB wrt a perfect short dipole which would have a gain of 2.15dB wrt isotropic so you should have -5.21dB.

Your NEC result seems plausible.

To match to 50 ohms with a shunt coil you would need to adjust the loading coils so that the feedpoint impedance is capacitive, equal to 50 ohms in parallel with some capacitance. Then add the shunt coil to cancel the capacitance. In effect, you have an L-match.
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W5DXP
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« Reply #2 on: April 15, 2013, 05:51:36 AM »

1. Does the above gain sound about right?

That would be an efficiency of 20% compared to a ground mounted 1/4WL vertical over mininec ground - sounds about right for an antenna about the size of a 40m mobile antenna. Center loading the two elements would probably increase the gain as the high feedpoint current would be flowing through one meter of copper instead of through loading coils.
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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.
RFRY
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« Reply #3 on: April 15, 2013, 06:17:20 AM »

Unfortunately, loading and running the original model in 4nec2 (NEC4.2 engine) produced these warnings:

7MHz Short Dipole.txt wavelength=42.83 mtr.

Warn.: Wire 1, seg 1 (tag 1), radius (0.1) differs more than 5 * radius (0.01) for wire 2
Warn.: Wire 1, seg 1 (tag 1), len/rad at junction (2.222) below 6.
Warn.: Wire 1, seg 9 (tag 1), radius (0.1) differs more than 5 * radius (0.01) for wire 4
Warn.: Wire 1, seg 9 (tag 1), len/rad at junction (2.222) below 6.
Warn.: Wire 3, seg 13 (tag 5), radius (0.01) differs more than 5 * radius (0.1) for wire 1
Warn.: Wire 5, seg 19 (tag 17), radius (0.01) differs more than 5 * radius (0.1) for wire 1


Modifying and simplifying the model per the data below removed all the warnings...

CM
CE
GW   1   19   0   0   0.5   0   0   2.5   0.01
GW   2   3   0   0   0.5   0   -1   0.5   0.01
GW   5   3   0   0   0.5   0   1   0.5   0.01
GW   14   3   0   0   2.5   0   -1   2.5   0.01
GW   17   3   0   0   2.5   0   1   2.5   0.01
GE   -1
LD   0   1   10   10   2   29.64e-6
GN   2   0   0   0   3   0.0001
EK
EX   0   1   9   0   1   0   0
FR   0   0   0   0   7   0
EN

I combined the loading inductances into one, and assumed it had 2 ohms of ESR.  I also assumed that the other conductors had negligible ohmic losses.

The system is resonant at 7 MHz, with a radiation resistance of 6.8 ohms.  All that remains is to use a network at the feedpoint to match the antenna to the Zo of the transmission line connected there.

Following is a link to a screen clip showing that for the ground defined in the original model, the system will radiate about 3.3 watts when 5 watts of Z-matched power is applied to the feedpoint, or an efficiency of about 67%.

The screen clip includes a plot of the field intensity produced in the elevation plane by that radiated power at an h-distance of 1 km.  It shows that the maximum radiated field at that h distance occurs at an elevation angle of about 25 degrees -- significantly lower than shown in a NEC far-field calculation.

That low-angle radiation will proceed to the ionosphere, and given the right conditions, be reflected to return to the earth as skywaves.

http://www.freeimagehosting.net/t4jxz

R. Fry

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JAHAM2BE
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« Reply #4 on: April 15, 2013, 07:17:24 AM »

Thank you, G8HQP, W5DXP, and RFRY. Your responses are extremely educational. Please allow me to follow up with a couple of more questions:

To match to 50 ohms with a shunt coil you would need to adjust the loading coils so that the feedpoint impedance is capacitive, equal to 50 ohms in parallel with some capacitance. Then add the shunt coil to cancel the capacitance. In effect, you have an L-match.

That makes sense. Some more playing with the 4nec2 optimizer seems to indicate that adjusting the loading coils to achieve a 50 ohm feedpoint R yields an inductive feedpoint reactance. I originally based my model on an N3OX short dipole model that used a shunt inductor for matching, but it seems that my altered model requires capacitance for matching.

Quote from: W5DXP
Center loading the two elements would probably increase the gain as the high feedpoint current would be flowing through one meter of copper instead of through loading coils.

One thing that is confusing me: is there an easy way to determine the position of the loading coil that requires the least inductance (hence minimizing inductor loss)? In running my simulations it seems (now) that the required inductance is minimum at the radiator ends, near the capacity hats. However, looking at another shortened dipole calculator (http://www.k7mem.com/Electronic_Notebook/antennas/shortant.html) the required inductance seems to vary non-monotonically as the distance from the feedpoint is varied.

Quote from: RFRY
Modifying and simplifying the model per the data below removed all the warnings... [...] The screen clip includes a plot of the field intensity produced in the elevation plane by that radiated power at an h-distance of 1 km.  It shows that the maximum radiated field at that h distance occurs at an elevation angle of about 25 degrees -- significantly lower than shown in a NEC far-field calculation.

Thank you for correcting the model. Might you expand upon how you created the field intensity plot in the elevation plane (the middle graph in your screen capture)? I have been exploring the menus in 4nec2 v5.8.11, but I couldn't seem to figure out how create a graph identical to yours. EDIT: Got it. It's the surface wave calculation, whose existence I wasn't aware of. Very nice. But what is the significance of the 1km distance chosen? Why not 0.5km, or 2km? And what is meant by "h-distance"?

Quote from: RFRY
That low-angle radiation will proceed to the ionosphere, and given the right conditions, be reflected to return to the earth as skywaves.

Excellent. Until now I've been experimenting with and constructing mainly small transmitting loop antennas, with their tiny radiation resistance attendant constructional difficulties. It's very encouraging to see that acceptably efficient low-angle radiation is also possible from a higher-radiation-resistance structure like a small vertical dipole near ground. They key will of course be minimizing the loading coil losses - an entire research subject in itself, from what I gather.
« Last Edit: April 15, 2013, 07:42:31 AM by JAHAM2BE » Logged

G8HQP
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« Reply #5 on: April 15, 2013, 08:24:33 AM »

You can match with shunt inductor or shunt capacitor, depending on the loading coils. Resonate above 7Mz and add an inductor, or below 7MHz and add a capacitor. Up to you. One of these might show very slightly better bandwidth, but the difference will be small.

Bear in mind that you could get further losses from whatever balun you use. Without a balun you may get better radiation from the feeder than from the antenna!
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WB6BYU
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« Reply #6 on: April 15, 2013, 08:50:56 AM »

Quote from: JAHAM2BE

I originally based my model on an N3OX short dipole model that used a shunt inductor for matching, but it seems that my altered model requires capacitance for matching.


Either will work, assuming that the loading inductors are adjusted accordingly.
I suspect your problem may be the resolution of the loading coil adjustment:
try a shunt value around 0.4uH and then adjust the loading coils for resonance.
If the resistance is too high at resonance, make the coil smaller.  That's probably
easier than using ~1300pf.

To match 5 ohms you need roughly 15 ohms of shunt reactance and a similar amount
of reactance from tuning the rest of the antenna slightly off frequency.  With a high
Q antenna you can quickly go from +15 ohms to -15 ohms, which changes the required
shunt element from a capacitor to a coil.



Quote

One thing that is confusing me: is there an easy way to determine the position of the loading coil that requires the least inductance (hence minimizing inductor loss)?



The loading coil requires minimum inductance at the feedpoint, and more inductance
as you move it out along the antenna.  But that doesn't mean that the efficiency
is best with the coil at the feedpoint, however, because moving the coil away from the
feedpoint increases the radiation resistance, and the ratio between that and the loss
resistance is what determines your efficiency.

My recommendation would be to double the wires on your capacity had and put the coils
between the hat and the center radiator.  Adding more capacity hat reduces the required
inductance, while putting the coil as far out as possible raises the radiation resistance.



Quote

They key will of course be minimizing the loading coil losses...



Not exactly:  you want to maximize efficiency, which requires consideration of
the radiation resistance.  Doubling your coil loss resistance can actually improve your
signal if your radiation resistance goes up by a factor of 4 in the process.
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RFRY
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« Reply #7 on: April 15, 2013, 09:18:49 AM »

But what is the significance of the 1km distance chosen? Why not 0.5km, or 2km? And what is meant by "h-distance"?

The distance is somewhat arbitrary.  It just needs to be somewhat past the near-field radius of the antenna system -- which in classic (but not NEC) definition ends at a distance of 2D2/lambda from the structure, where D is the aperture of the antenna.

An h-distance (horizontal distance) of 1 km commonly is used when evaluating the fields of single monopoles or dipoles used in broadcasting.

When a ground plane is defined in NEC, the far field patterns it calculates are based on an infinite distance from the radiator, over an infinite, flat surface.  But radiation patterns at much shorter distances have much different shapes in the real world, and a significant amount of the radiation from an antenna can go unrecognized when looking only at a NEC far-field pattern.  This was illustrated in the screen clip I posted earlier in this thread.

RF
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W5DXP
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« Reply #8 on: April 15, 2013, 06:03:23 PM »

is there an easy way to determine the position of the loading coil that requires the least inductance (hence minimizing inductor loss)?

As Dale says, one needs to worry about radiation resistance, not least inductance. I once won a 75m mobile shootout contest using a CB whip with a coil at the top end of the whip followed by a top hat. It took a lot of inductance but there wasn't much current flowing through it.

The farther away from the feedpoint one puts the loading coil, the more inductance is required along with more wire and more resistance. But the farther away from the feedpoint one puts the loading coil, the lower the current flowing through it and since losses are proportional to the square of the current, the losses are usually lower for the higher inductance required for center loading.
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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.
JAHAM2BE
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« Reply #9 on: April 15, 2013, 08:10:47 PM »

I suspect your problem may be the resolution of the loading coil adjustment:
try a shunt value around 0.4uH and then adjust the loading coils for resonance.
If the resistance is too high at resonance, make the coil smaller.  That's probably
easier than using ~1300pf.

I continued to play with matching within 4nec2 but no matter what the shunt coil value, the resistive portion of feedpoint impedance never changes.

I'm beginning to suspect my method of modeling the shunt coil (beta match) is inappropriate so that the coil value is somehow not properly affecting the resistive portion of feedpoint impedance. I simply declared a lumped RLC assigned to the same segment as the feedpoint (source).

I'll continue to investigate with the assumption that I have made a modeling error. If anyone could give an example of how how to properly model a shunt coil/beta match in NEC, that would help.
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N3OX
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« Reply #10 on: April 16, 2013, 07:42:31 AM »

I'm beginning to suspect my method of modeling the shunt coil (beta match) is inappropriate so that the coil value is somehow not properly affecting the resistive portion of feedpoint impedance. I simply declared a lumped RLC assigned to the same segment as the feedpoint (source).

You need to make sure it's assigned to be in PARALLEL with the source. I haven't messed around with bare NEC-2 or 4nec2 enough to know how to do this but I think there's a different setting or flag or whatever to put two objects on the same segment in parallel instead of series.

-----------

For modeling purposes you can also calculate the required shunt inductance exactly (or nearly so) from the modeled resistance to help you dial in the right values. The feedpoint impedance Zf = Rf+jXf with NO shunt should be adjusted so that it presents the following reactance Xf (this is usually easy enough to do as long as Rf doesn't change much at the desired operating frequency, otherwise it will take iteration), and then you should place a shunt reactance Xs also calculated below across the feedpoint.

For feedpoint resistance Rf:  
1)Calculate delta = sqrt(50/Rf-1)
2)Adjust the loading inductors/cap hats such that Xf = -delta*Rf
3)Then calculate the shunt reactance Xs = +50/delta

The parallel combination of Zf and Xs should then be 50+j0. If you want something other than 50 ohms, replace the "50"s above with the desired feed resistance when matched.

----------------

Example: say Zf = 9+jXf where Xf can be adjusted by tuning with no shunt.
Xf = -9*sqrt(50/9-1) = -19.2 ==> Zf = 9-j19.2
Xs = +50/sqrt(50/9-1) = +23.43

When in parallel, Zmatched = Xs||Zf

Zmatched = 1/(1/(9-j*19.2) + 1/(j*23.43)) = 49.96-j0.05 ( http://goo.gl/LemgY )
« Last Edit: April 16, 2013, 07:44:53 AM by N3OX » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
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« Reply #11 on: April 16, 2013, 08:23:10 AM »

Quote from: JAHAM2BE

I'm beginning to suspect my method of modeling the shunt coil (beta match) is inappropriate so that the coil value is somehow not properly affecting the resistive portion of feedpoint impedance. I simply declared a lumped RLC assigned to the same segment as the feedpoint (source).



In my experience with EZNEC, that puts the coil in series with the feedpoint rather
than shunt, which is consistent with your observation that R doesn't change.

There might be a flag somewhere in the software, but what I end up having to do
is to add wire segments to form a square, with the feedpoint on one side and the
shunt element on the other side.  You could also do this by making a triangle with
the top connected to the vertical section, the other two corners to ground, with
the shunt element in one leg and the feedpoint in the other.  Due to the small
values of inductance required, however, this doesn't give an exact result because
of the inductance and/or transmission line effects of the wire segments.

The alternate is to use the standard formula for an "L" network and calculate the
required shunt and series reactances to match the feedpoint resistance to 50 ohms.
The number I provided earlier came from one of W4RNL's spread sheets, which is
a convenient way to implement this.
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JAHAM2BE
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« Reply #12 on: April 16, 2013, 09:23:11 AM »

Another feeding question: is it possible to feed the antenna at the end, like an end-fed half wave antenna? This could vastly simplify mechanical construction. Imagine the following: EFHW tuner at one end of the dipole, followed by the capacity hat, one loading coil, then the unbroken 2m radiator, followed by the opposite loading coil and finally the opposite capacity hat. The fact that the radiator is unbroken makes it sturdier than if it were split in two and fed at the center.

So is end-feeding possible with a capacity-hatted, loaded dipole?   
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WB6BYU
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« Reply #13 on: April 16, 2013, 09:47:43 AM »

It is possible, yes, but I would instead consider an off-center feed:  that is,
feed it at the junction of the capacity hat and the center conductor rather
than at the end of the hat (which will cause some unequal current distribution
in the hat wires.)

This can be on either side of the loading coil if you put it at that point.  In
fact, you can inductively couple the coax to the loading coil, which may
help to reduce problems with common mode current.

Another option that might be worth considering is to use a folded element
with two separate conductors for the center portion, joining each other at
the junction with the capacity hat.  Then you can run the coax up through
the main conductor and out through a hole to a break in the parallel
conductor, giving an impedance step-up function. 

Any of these methods will require very good feedline decoupling - possibly
two chokes spaced 1/4 wave apart - to reduce common mode current.
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JAHAM2BE
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« Reply #14 on: April 16, 2013, 07:18:58 PM »

It is possible, yes, but I would instead consider an off-center feed: [..] In
fact, you can inductively couple the coax to the loading coil, which may
help to reduce problems with common mode current.

Very interesting. Since the impedance at the loading coil (at the extremity of the radiator) is high, does this mean that just winding a few turns of wire on top of the loading coil, to achieve an impedance step down from the antenna to the coax, would be sufficient? The coax would then just connect directly to the coupling coil?

I must admit that I don't have a clear idea of the complex impedance visible at the loading coil, and I have no experience with matching networks except for the simple inductive coupling loop used in small transmitting loops. When I get home I'll see if I can figure out the inductive coupling feed method in 4nec2, but any pointers in the right direction would be appreciated.
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