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OT antenna question

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Michael Blake

OT antenna question

While off topic this antenna supports an all Elecraft station.

I have very limited antenna space and my 80M/20M shortened dipole consists of 17’ center sections connected to approximately 44uH, 2.5” OD inductors with 10’ end wire sections past the inductors and terminated in end insulators.

The center sections of the dipole are tuned for resonance at about 14.15 MHz and the end sections, past the loading inductors on each dipole leg, are tuned for resonance a 3.7 MHz. The resonance dips at both frequencies are quite sharp and the SWR on both bands is less than 1.5 to 1.

The question that I can not get my head around is this. On 20M the two inductors are acting as chokes and not traps but I have no idea what the losses might be with these chokes acting as end insulators rather than a trap or actual insulator.

In actual operation the antenna works quite well on both bands but I would like to get an idea on how efficient these inductors are when used as chokes (4,000 ohms) on 20M.

Very 73 - Mike - K9JRI

K3s - P3 - KPA500
KX3 - KPA500

Michael Blake
[hidden email] <mailto:[hidden email]>

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John Oppenheimer

Re: OT antenna question

Hi Mike,

An EZNEC model using 30 feet AGL and Q=200 for the coils, has a very
close match the the reported values.

20M, the antenna does not seem to degrade form a simple dipole with a
Gain of about 6 dBi at about 30 degrees elevation.

80M, it's about 5 dBi at 90 degrees elevation. Expected from a low
dipole. The impedance at resonance is about 25 Ohm, so should have a SWR
about 2. A lower measured SWR may be the result of other system loses.

I'd suggest this antenna is a good option for the constraints imposed.

John KN5L

On 05/28/2018 11:39 AM, Michael Blake wrote:
> The center sections of the dipole are tuned for resonance at about 14.15 MHz and the end sections, past the loading inductors on each dipole leg, are tuned for resonance a 3.7 MHz. The resonance dips at both frequencies are quite sharp and the SWR on both bands is less than 1.5 to 1.
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Michael Blake

Re: OT antenna question

Thank you very much John.

Very 73 - Mike - k9JRI

> On May 28, 2018, at 1:39 PM, John Oppenheimer <[hidden email]> wrote:
>
> Hi Mike,
>
> An EZNEC model using 30 feet AGL and Q=200 for the coils, has a very
> close match the the reported values.
>
> 20M, the antenna does not seem to degrade form a simple dipole with a
> Gain of about 6 dBi at about 30 degrees elevation.
>
> 80M, it's about 5 dBi at 90 degrees elevation. Expected from a low
> dipole. The impedance at resonance is about 25 Ohm, so should have a SWR
> about 2. A lower measured SWR may be the result of other system loses.
>
> I'd suggest this antenna is a good option for the constraints imposed.
>
> John KN5L
>
> On 05/28/2018 11:39 AM, Michael Blake wrote:
>> The center sections of the dipole are tuned for resonance at about 14.15 MHz and the end sections, past the loading inductors on each dipole leg, are tuned for resonance a 3.7 MHz. The resonance dips at both frequencies are quite sharp and the SWR on both bands is less than 1.5 to 1.
> ______________________________________________________________
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Jim Brown-10

Re: OT antenna question

In reply to this post by John Oppenheimer

On 5/28/2018 10:39 AM, John Oppenheimer wrote:
> An EZNEC model using 30 feet AGL and Q=200 for the coils, has a very
> close match the the reported values.

While SWR and driving Z values may be correct, there was an excellent
piece 2-part piece in QEX 3-4 years ago showing that NEC does not
correctly model phase shift through inductors, thus producing wrong
answers for field strength (i.e., how well the antenna WORKS). This is a
shortcoming of the NEC code, and is unrelated to the user interface like
EZNEC/ELNEC and 4NEC2.

The antenna he was studying was a loaded mobile whip for HF. The first
part of the article was a serious engineering description of the test
setup, including the measurement system. The second described a
carefully controlled study, using that measurement system, of inductive
loading at the base, midway, and fairly high on the vertical radiator,
as well as capacitive top loading. He compared measured results with the
NEC model. As I recall, differences between modeled and measured data
were as much as 10-14 dB for bottom loading.

In addition to exposing the limitation of NEC, the major conclusion from
his study was that because radiation is produced by current and current
is greatest near the feedpoint, a design that provides the greatest
vertical height before the loading element(s) will generally be the most
efficient (that is, produce the loudest signal). Those who have long
been serious about HF mobile antennas have known this for years. At
least as long as 20 years ago, W8JI has written a lot about this. In
that time frame, my buddy K9IKZ told me about an annual conclave of HF
mobile station builders convened in southern Indiana to do "shootouts"
of their designs.

73, Jim K9YC

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Clay Autery-2

Re: OT antenna question

Mmmm... are you saying that NEC2 handles the phsae shift through
inductors correctly/better?

"Looking for reasons to pay the license fee for NEC2)

73,

______________________
Clay Autery, KY5G
(318) 518-1389

On 05/28/18 15:18, Jim Brown wrote:
> and is unrelated to the user interface like EZNEC/ELNEC and 4NEC2

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David Gilbert

Re: OT antenna question

There is no license fee for NEC2, and EZNEC and 4NEC2 both use it freely.

NEC4 is another matter ...

Dave AB7E

On 5/28/2018 4:19 PM, Clay Autery wrote:

> Mmmm... are you saying that NEC2 handles the phsae shift through
> inductors correctly/better?
>
> "Looking for reasons to pay the license fee for NEC2)
>
> 73,
>
> ______________________
> Clay Autery, KY5G
> (318) 518-1389
>
> On 05/28/18 15:18, Jim Brown wrote:
>> and is unrelated to the user interface like EZNEC/ELNEC and 4NEC2
>
> ______________________________________________________________
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>

Clay Autery-2

Re: OT antenna question

My mistake... doh! I meant 4.

Increment version by 2... same question... Does NEC-4 handle the phase
shift through inductors correctly?

It's a big chunk of change to get the NEC-4 license and upgrade the
front end license... need to compile multiple reasons to justify it.
<wink>

Appreciate the catch...

______________________
Clay Autery, KY5G
(318) 518-1389

On 05/28/18 18:36, David Gilbert wrote:

>
> There is no license fee for NEC2, and EZNEC and 4NEC2 both use it freely.
>
> NEC4 is another matter ...
>
> Dave AB7E
>
>
>
> On 5/28/2018 4:19 PM, Clay Autery wrote:
>> Mmmm... are you saying that NEC2 handles the phsae shift through
>> inductors correctly/better?
>>
>> "Looking for reasons to pay the license fee for NEC2)
>>
>> 73,
>>
>> ______________________
>> Clay Autery, KY5G
>> (318) 518-1389
>>
>> On 05/28/18 15:18, Jim Brown wrote:
>>> and is unrelated to the user interface like EZNEC/ELNEC and 4NEC2
>>
>> ______________________________________________________________
>> Elecraft mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:[hidden email]
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>> Message delivered to [hidden email]
>>
>
> ______________________________________________________________
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k6dgw

Re: OT antenna question

In reply to this post by Clay Autery-2

Clay,

NEC-2 is public domain. NEC-4 is not, it is licensed and not free. I
believe that all flavors of EZNEC Ver 4.0 EXCEPT EZNEC/4 use the NEC-2
engine. NEC-2, as a simulator, has some deviations from reality ...
inductor modeling is one of them, conductors connected at small angles,
traps [see inductors] and modeling buried radial fields are others.
NEC-4 is significantly more complex, more faithful to reality, and not free.

The native UI to NEC-2 is not friendly, it's origin dates to 80-column
punched cards. EZNEC is a windows UI to it.

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 5/28/2018 4:19 PM, Clay Autery wrote:

Jim Brown-10

Re: OT antenna question

In reply to this post by Clay Autery-2

On 5/28/2018 5:03 PM, Clay Autery wrote:
> Does NEC-4 handle the phase shift through inductors correctly?

I doubt it, but I don't know. I believe the issue is the result of the
computational model.

73, Jim K9YC

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Wes Stewart-2

Re: OT antenna question

In reply to this post by Clay Autery-2

I hope you mean NEC4.

Wes N7WS

On 5/28/2018 4:19 PM, Clay Autery wrote:
> Mmmm... are you saying that NEC2 handles the phsae shift through inductors
> correctly/better?
>
> "Looking for reasons to pay the license fee for NEC2)
>
> 73,

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k6dgw

Re: OT antenna question

In reply to this post by Clay Autery-2

Well ... define "correctly." All simulation is an approximation of
reality. NEC-4 adds capabilities such as buried radial fields, and is
more faithful for tiny antennas than NEC-2. It treats lumped circuit
constants pretty much the same as NEC-2, however.

Last time I checked, a US non-commercial NEC-4 license was $300, IIRC.
You'd need the NEC-4 EZNEC UI too. Since the vast majority of
environment variables impacting antenna performance in the real world
are not part of any of the models, and we are "amateurs," I doubt you
would see any benefit from NEC-4.

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 5/28/2018 5:03 PM, Clay Autery wrote:

> My mistake... doh! I meant 4.
>
> Increment version by 2... same question... Does NEC-4 handle the phase
> shift through inductors correctly?
>
> It's a big chunk of change to get the NEC-4 license and upgrade the
> front end license... need to compile multiple reasons to justify it.
> <wink>
>
> Appreciate the catch...
>
> ______________________
> Clay Autery, KY5G
> (318) 518-1389
>