[TowerTalk] 160 vertical question (Top Hat)

[Jim Brown]

I agree with both Grant and Rick, but with this caveat.  Current is 
greatest near the feedpoint, and an inductor at the feedpoint is right 
at the current peak. By contrast, current is least near the end of the 
antenna. SO -- this is an important reason why top loading is better 
than inductive loading. BTW -- there was an excellent piece of work 
published 3-4 years ago studying the relative effectiveness of bottom, 
center, and top loading of mobile antennas. The study included extensive 
measurements of field strength, which confirmed that top loading was 
significantly better, AND that NEC failed to accurately model inductive 
loading.  As I understand it, NEC models the inductor is as a lumped 
component, so fails to accurately take into account phase shift across it.

Another technique that is often used for matching is to make the wires 
much longer than resonant (that is, resonant around the top of the BC 
band), so that the feedpoint Z is 50 +jX ohms (that is, inductive) on 
160M, and tune the capacitance out by adding a capacitor of equal value 
in series.    NEC can easily model this.

73, Jim K9YC

On 2/3/2018 7:51 PM, Grant Saviers wrote:
> Top loading is a winner to increase antenna efficiency.  I agree that 
> EZNEC is essential since umbrella (slanted) wires have less effect 
> than a flat top hat and also tend to shield the antenna a bit. The 
> model needs the segments aligned in each element.  Trig or use AutoEZ 
> to figure that out.
>
> There are a couple of articles worth reading on Rudy Severns web site 
> antennasbyn6lf.com
>
> This article is for top loaded verticals for the new LF bands but 
> covers the principles well
> http://rudys.typepad.com/files/chapter-3--1.pdf
>
> My T is 10' at the base elevated radials, 85' to the top, and 33' or 
> so each side, resonates at 1820, 25 ohms.
>
> Grant KZ1W
>
>
> On 2/3/2018 17:44 PM, Richard (Rick) Karlquist wrote:
>> You should really model this on EZNEC.
>> Having done this many times, I can say
>> that the length of the vertical plus the
>> length of one of the top wires should be
>> about 120 feet to hit exact 1830 resonance
>> without a loading coil.  However,
>> there is absolutely no compelling reason why
>> this is necessary.  A loading/matching coil
>> is far more convenient if you keep the wire
>> length to 25 feet.  Therefore, it should
>> never be necessary to put it up and down a
>> bunch of times to get it "just right".  There
>> is no "just right" with respect to the antenna
>> proper.  Use the taps on the coil to do the trimming
>> to make the system just right.  This has been
>> discussed many times on this reflector.
>>
>> Rick N6RK
>>
>> On 2/3/2018 5:20 PM, Bill via TowerTalk wrote:
>>>
>>> Planning on putting up a 160 vertical, but of course a 1/4 wave 
>>> vertical is out of the question here in HI since we have a height 
>>> limitation.
>>>   Here's the question:  I'm putting up around 50 feet of 3 inch 
>>> aluminum tubing  (left over from various antenna takedowns).  I plan 
>>> on putting up on top of that around another 10 or 15 feet of smaller 
>>> diameter stinger for a total height of around 60-65 feet.  I 
>>> obviously need to make up for the lack of height and I was planning 
>>> on making a top hat of four wires coming down from the top of the 
>>> antenna at around a 45 degree angle.  The problem is how long to 
>>> make each of those wires.  My guess is around 25 feet based on my 
>>> past experience in making Tee-topped 80 meter verticals.  However, 
>>> it is only a guess and I don't feel like yanking the 3 inch pipe up 
>>> and down a bunch of times to get it just right.
>>>   I know some trimming will be necessary since this is not an exact 
>>> science.  I just need an educated guess for where to start with the 
>>> length.  Any ideas?
>>>   Bill K4XS/KH7XS
>>> _______________________________________________
>>>
>>>
>>>
>>> _______________________________________________
>>> TowerTalk mailing list
>>> TowerTalk at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>>
>>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>