[Antennas] help

ik0 ik0clm at libero.it
Tue Nov 27 13:23:17 EST 2012 


>----Messaggio originale----
>Da: antennas-request at mailman.qth.net
>Data: 27/11/2012 18.00
>A: <antennas at mailman.qth.net>
>Ogg: Antennas Digest, Vol 103, Issue 5
>
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>Today's Topics:
>
>   1. Re: Next dumb question (Terry Conboy)
>   2. Re: Next dumb question (C.Whitaker)
>
>
>----------------------------------------------------------------------
>
>Message: 1
>Date: Mon, 26 Nov 2012 11:01:31 -0800
>From: Terry Conboy <n6ry at arrl.net>
>To: antennas at mailman.qth.net
>Subject: Re: [Antennas] Next dumb question
>Message-ID: <50B3BC8B.7030205 at arrl.net>
>Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
>Indeed, a large enough capacity hat can reduce or totally eliminate the 
>need for a loading coil.  Use of capacity hats can increase the 
>bandwidth and radiation resistance significantly, which can improve 
>efficiently for a given ground loss resistance.  One thing to be careful 
>of is to not to mount any loading coil too close the the hat, or the 
>added parallel capacitance across the coil can increase circulating 
>currents in the coil and lower the effective coil Q (increase losses). 
>Probably a good rule of thumb is to space it away from the hat by at 
>least by the length of the coil.
>
>Also, if the hat conductors slope down from horizontal, the fields can 
>partially cancel the radiation from the vertical conductor which will 
>slightly lower the radiation resistance.  Sometimes adding more hat 
>wires of shorter length is a viable solution, if you are stuck with 
>supports that give a similar droop angle. Obviously, there are tradeoffs 
>for maximum efficiency between coil size and making drooping hat wires 
>longer.
>
>Generally, it's a good idea to keep the capacity hat conductors 
>symmetrical around the vertical.  Any asymmetry can result in high angle 
>horizontally polarized signals, which can be good or bad, depending on 
>desired path lengths.  (The inverted-L can be thought of as a single 
>wire hat, and it can produce significant NVIS signals.)
>
>73, Terry N6RY
>
>On 2012-11-23 7:28 PM, fred forbush wrote:
>> Concerning the cap. hat the way I understand it is that the electrostatic
>> field surrounding the hat interacts with the electromagnetic field of the
>> loanding coil beneath it. I've found that using a cap. hat reduces the
>> amount of coil needed when tuning to a frequency.
>> 73, Fred
>> k6kub
>>
>>
>> On Fri, Nov 23, 2012 at 11:08 AM, Michael Josefsson <mj at isy.liu.se> wrote:
>>
>>> On 23 Nov 2012, at 18:39, Jim Miller wrote:
>>>
>>>> Is a capacity hat electrically connected (metal to metal) to the antenna
>>>> itself or is it just sitting there insulated but "capacitively coupled"?
>>>>
>>> It is electrically connected.
>>>
>>>> The coil I bought was supposed to be for 15 to 80 meters.  Well.. It 
only
>>>> tunes down (no coil) to about 4.1 and SWR is too high to use even at
>>> 3.963
>>>> MHz (and it won't go into the 15 meter band either, 20 is tops).  OK,
>>>> shorter whip for 15.
>>>>
>>>> But, for 80 meters, thinking about stationary operation and adding a
>>>> temporary top hat to bring me into the top end of the band.
>>>>
>>>> Better yet, also thinking about adding a few turns of extra coil at the
>>> base
>>>> to bring the whole thing down a bit.  How much would it take me to move
>>> from
>>>> 4.1 down to 3.9 or so using existing coil for adjustment and my 102 in
>>> whip?
>>>> Say #12 THHN close wound on a 2 inch form (6,8,?)
>>>>
>>> Everything depends on everything else... Trial and horror is your path :/
>>>
>>> Cheers anyway:)
>>>
>>> /Micke
>>>
>
>
>
>------------------------------
>
>Message: 2
>Date: Tue, 27 Nov 2012 07:34:44 -0500
>From: "C.Whitaker" <whitaker at pa.net>
>To: antennas at mailman.qth.net
>Subject: Re: [Antennas] Next dumb question
>Message-ID: <50B4B364.8090106 at pa.net>
>Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
>de WB2CPN
>For those of  you who live in North New Jersey, in sight
>of NYC, there was, or is, a group of four radio towers
>which have hats on them.  I understand they were used
>for commercial oversees radio circuits.  Maybe McKay (?)
>or Federal.  (Armstrong's FM test site is a bit further
>up the road, in Alpine, NJ.).  (Also, Federal's TACAN
>aircraft navigation test facility was on Route 3 in Nutley, NJ.
>New Jersey had a lot going for it back then.
>73  Clete
>+++++++++++++++++++++++++++++++++++++++++
>On 11/26/2012 2:01 PM, Terry Conboy wrote:
>> Indeed, a large enough capacity hat can reduce or totally eliminate 
>> the need for a loading coil.  Use of capacity hats can increase the 
>> bandwidth and radiation resistance significantly, which can improve 
>> efficiently for a given ground loss resistance.  One thing to be 
>> careful of is to not to mount any loading coil too close the the hat, 
>> or the added parallel capacitance across the coil can increase 
>> circulating currents in the coil and lower the effective coil Q 
>> (increase losses). Probably a good rule of thumb is to space it away 
>> from the hat by at least by the length of the coil.
>>
>> Also, if the hat conductors slope down from horizontal, the fields can 
>> partially cancel the radiation from the vertical conductor which will 
>> slightly lower the radiation resistance.  Sometimes adding more hat 
>> wires of shorter length is a viable solution, if you are stuck with 
>> supports that give a similar droop angle. Obviously, there are 
>> tradeoffs for maximum efficiency between coil size and making drooping 
>> hat wires longer.
>>
>> Generally, it's a good idea to keep the capacity hat conductors 
>> symmetrical around the vertical.  Any asymmetry can result in high 
>> angle horizontally polarized signals, which can be good or bad, 
>> depending on desired path lengths.  (The inverted-L can be thought of 
>> as a single wire hat, and it can produce significant NVIS signals.)
>>
>> 73, Terry N6RY
>>
>> On 2012-11-23 7:28 PM, fred forbush wrote:
>>> Concerning the cap. hat the way I understand it is that the 
>>> electrostatic
>>> field surrounding the hat interacts with the electromagnetic field of 
>>> the
>>> loanding coil beneath it. I've found that using a cap. hat reduces the
>>> amount of coil needed when tuning to a frequency.
>>> 73, Fred
>>> k6kub
>>>
>>>
>>> On Fri, Nov 23, 2012 at 11:08 AM, Michael Josefsson <mj at isy.liu.se> 
>>> wrote:
>>>
>>>> On 23 Nov 2012, at 18:39, Jim Miller wrote:
>>>>
>>>>> Is a capacity hat electrically connected (metal to metal) to the 
>>>>> antenna
>>>>> itself or is it just sitting there insulated but "capacitively 
>>>>> coupled"?
>>>>>
>>>> It is electrically connected.
>>>>
>>>>> The coil I bought was supposed to be for 15 to 80 meters.  Well.. 
>>>>> It only
>>>>> tunes down (no coil) to about 4.1 and SWR is too high to use even at
>>>> 3.963
>>>>> MHz (and it won't go into the 15 meter band either, 20 is tops).  OK,
>>>>> shorter whip for 15.
>>>>>
>>>>> But, for 80 meters, thinking about stationary operation and adding a
>>>>> temporary top hat to bring me into the top end of the band.
>>>>>
>>>>> Better yet, also thinking about adding a few turns of extra coil at 
>>>>> the
>>>> base
>>>>> to bring the whole thing down a bit. How much would it take me to move
>>>> from
>>>>> 4.1 down to 3.9 or so using existing coil for adjustment and my 102 in
>>>> whip?
>>>>> Say #12 THHN close wound on a 2 inch form (6,8,?)
>>>>>
>>>> Everything depends on everything else... Trial and horror is your 
>>>> path :/
>>>>
>>>> Cheers anyway:)
>>>>
>>>> /Micke
>>>>
>>
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