[Antennaware] Help with Short Dipole

Mon Mar 4 19:53:53 EST 2019

Hello Guy. You have given me a tremendous amount of excellent 
information which is awesome. Thank you so much. I will try to build a 
model in my EZNEC 5.0 (standard NEC-2 engine version) and then submit it 
to the group to see how close I came to your description& suggestions. 
At this point I would love to have the EZNEC Pro version running with 
the NEC-4 engine but that is beyond my financial resources at the moment.

At this point two questions......can you suggest what font I should 
select in my e-mail program such that your diagram is lined up 
correctly?  Right now it is badly scattered all over and it is hard for 
me to visualize and correlate it with your words.

Second question, I understand 100% what you are saying about the 
feedline as being part of the antenna system from a radiation point of 
view, and I will be for sure adding several good common mode chokes per 
Jim's excellent tutorial in the final build but I think at this point or 
stage I would like to simply use the coax as a means to model the 
feedpoint and coils together and to see what the connections will look 
like in the model and the theoretical impedance at the shack end of the 
coax if the feedline were not part of the radiating element. Once I have 
that right then I would like to advance to a more final model that takes 
into account the coax shield etc. Would that be ok with you? I fear this 
level of modeling may be a little over my head BUT I will give it a try 
hi-hi.

I realize that in some situations the impedance at the antenna feedpoint 
can be close to that measured in the shack after the run of coax and a 
lot depends on the loss of the coax, and the impedance of the antenna, 
and of the coax, etc, etc.  But again, for right now, at least for this 
very beginning can we include the coax and it's connections but not 
treat it like a radiating element, only a connecting element with 50 ohm 
impedance.

If that is ok then how would that change your instructions you provided 
to me? Should I proceed with building up the model using the "box with 2 
wings" ?

Thanks!

Gedas, W8BYA

Gallery at http://w8bya.com
Light travels faster than sound....
This is why some people appear bright until you hear them speak.

On 3/4/2019 5:36 PM, Guy Olinger K2AV wrote:
> Hi Gedas,
>
> If I have read you correctly......
>
> First, set your ground descriptions to "very poor" or "extremely 
> poor". In reality, ground losses on 630m will be huge and unavoidable, 
> so one might as well take advantage of it in the model. You are only 
> three HUNDREDTHS of a wavelength above ground, roughly the same as 
> your 40 meter dipole was 4 feet off the ground. How well-performing, 
> and how decoupled with ground would you expect that to be.
>
> For this one, you MUST include the feedline as a conductor the same 
> diameter as the feedline stripped of jacket, plus insulation same type 
> and thickness as coax jacket.
>
> Then you will need to do something serious in reality to block the 
> feedline shield, which otherwise will become the principal radiator, 
> radiating 90% or more of the power. If you don't then you have to 
> include in your model every conductor connected to the coax shield, 
> transceiver chassis, table ground buss, including the ground itself, 
> only done reasonably in NEC4. You will most likely need a double 
> precision engine to get it to calculate right.
>
> I can have very short segments that the program calls "too short" for 
> the feed. That is done with the long wire either side of center then 
> with THREE very short wires (1 foot is good) between the two long 
> ones. Complete a square box by putting three more wires the same 
> length as the center short wire. You can think of it as a box with two 
> short wings coming off the top of the box.
>
> Make wires 2,3,4,6,7,8 all three segments. Set segments in 1 and 5 
> long enough to turn off the warning.
>
>         wire 1      2  3 4           5
> ===================+==+==+==+======================
>                     6 |  |7
> +==+
> | 8
>                     W |
>                     i |
>                     r |
>                     e |
> |
>                     9 |
> |
>
> Put the source in wire 8 center segment, and a part of the coil as a 
> load in wires 2,3,4. Adjust the size and ratios of coils in 2,3,4 for 
> resonance at 50 ohms at the source in wire 8.
>
> Wire 9 is the coax shield. Segment 1 gets a load that is the coax 
> block.  Wire 9 last segment, set a load in it to 25 ohms and connect 
> it to ground.  You can detect the model's sensitivity to grounding if 
> varying the R in the last segment load significantly changes the 
> source reading from wire 8.
>
> You screw around with the R and L  in wire 2,3,4 turns to match 
> reality for the R in a coil of X turns producing a certain uH. and 
> there's nothing that says that L in wire 2 load has to be the same as 
> wire 4.
>
> As to reality, a good common mode choke will be REQUIRED for your 
> actual results to make any sense at all. Read and reread 
> http://k9yc.com/630MTXChokes.pdf until you know it cold. Make it using 
> his specs. Don't screw around with his specifications. He's very 
> thorough and has already done the screwing around for you in advance 
> and measured the results.
>
> His graphs will allow you to create additions to the model which use 
> his MEASUREMENTS on the choke, which you can put in a load in the 
> first (top) segment of wire 9.
>
> A note, based on tough experience with fringe problems where ground is 
> important, use of NEC2 (especially single precision engines) can have 
> scattered killer issues that only go away when you pay the price, and 
> get EZNEC Pro4 and the NEC4 license. It's less than you would pay for 
> a third tier transceiver, and W7EL will take your questions because he 
> knows you're serious. BTW, Roy's help doc in the Pro4 version has 
> gotten really good over the years and is always the first place to go. 
> If it ain't working like you think it should, FIRST thing go dig in 
> his help doc. I may have preserved an entire 24/7 month or two of my 
> life doing that.
>
> Hope this has been of some help.
>
> 73, Guy K2AV
>
>
>
>
>
>
> On Mon, Mar 4, 2019 at 4:01 PM Gedas <w8bya at mchsi.com 
> <mailto:w8bya at mchsi.com>> wrote:
>
>     I was wondering if I could get some help developing then modeling a
>     short dipole for 630m use.
>
>     I consider myself to have intermediate modeling skills and have
>     modeled
>     a lot of antennas in EZNEC etc.
>
>     Here is what I would like to do. Take an existing 160m dipole
>     (flat top)
>     at 70' and use a pair of inductors, one on each leg about 1-2 feet
>     away
>     from the center insulator. I have already modeled this and so far
>     things
>     seem normal. My model was simplistic and did not include any coax.
>     For
>     the loads I simply increased the number of segments to 35 in each leg
>     such that the load ended up being within several % away from the
>     feedpoint. I had specified that the loads be at the ends (0%) but in
>     reality EZNEC put them several % out.
>
>     Now we all know given this tiny antenna for this band the feedpoint
>     impedance is going to be low.....in the ball-park of 12 - J30 is
>     about
>     the best I could achieve for the lowest SWR at 474 kHz. Not quite
>     resonant but close enough.
>
>     Now, I seem to remember an old trick that I saw many years ago
>     which I
>     think used to be called a gamma coil at the center of the dipole,
>     basically across the coax right at the feedpoint that will help bring
>     the feedpoint impedance up to 50 ohms. Similar to what many mobile
>     whip
>     antennas have at their feedpoint.
>
>     So I guess I have several questions that I need help with. One is
>     help
>     figuring out what this center inductor looks like in reality, what
>     value
>     it should be, _and how is it to be connected to the dipole and the
>     coax_. And two, how to model such a beast along with, say, 150' of
>     RG-8
>     coax.
>
>     In my basic model I simply used one segment for the center and is
>     where
>     the feedpoint is within EZNEC, then I have two additional wires
>     connected off this short center wire, one on each side of that center
>     segment that makes up the actual dipole.
>
>     Because of the frequency each segment has to be nearly 2' long ! This
>     too is where I need help and that is in attaching not only my coax
>     feed
>     but this center inductor in the model.  I hope the above makes
>     sense. TU
>
>     Gedas, W8BYA
>
>     Gallery at http://w8bya.com
>     Light travels faster than sound....
>     This is why some people appear bright until you hear them speak.
>
>
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