[TowerTalk] Loading Coils

[Richard (Rick) Karlquist]

Medhurst also developed a formula for the Q of an ideal inductor, which 
depends on diameter (8.5 inch) and the square root of frequency in MHz. 
If the turns are spaced around 1 tubing diameter apart, the Q will be 
about as high as it can get.

Using the Medhurst formula predicts a Q of 1.27 X 100 x 8.5 X sqrt(1.8)
= 1448.  This value is extremely difficult to accurately measure.  I 
certainly would not trust a VNA to do it, unless the coil is resonated 
with a very high quality capacitor at the measurement frequency.  You 
then set up the VNA to measure GROUP DELAY at resonance.  This number 
can be converted to Q.  I used to measure Q with an old HP8753 this way. 
  It's still a pretty good VNA even though it's been around for 40 years.

I have measured many coils and the Medhurst formula compares very 
closely with the measurements.  I have never seen a coil that has a Q 
that exceeds the Medhurst value.


73
Rick N6RK-

On 8/3/2025 8:08 AM, W7TMT - Patrick wrote:
> Regarding the need for any type of plating over copper, Greg, W8WWV had written extensively about building high Q coils and has also done an analysis of the coil characteristics after one has been in outdoor service for years.
> 
> His build article is here:
> 
> http://seed-solutions.com/gregordy/Amateur%20Radio/Experimentation/HiQCoil.htm
> 
> His follow up, including the testing he did of a coil with long outdoor exposure and its electrical characteristics before and after cleaning can be found here:
> 
> http://seed-solutions.com/gregordy/Amateur%20Radio/Experimentation/HiQCoil2.htm
> 
> 73
> Patrick, W7TMT
> 
> -----Original Message-----
> From: TowerTalk <towertalk-bounces at contesting.com> On Behalf Of Richard (Rick) Karlquist
> Sent: Sunday, August 3, 2025 07:50
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Loading Coils
> 
> WOW!  Many thanks to Wes for finding this obscure, but extremely valuable paper and posting it to TT.  Whenever Wes posts here, I make sure to read the post.  This paper completely debunks huge amounts of folklore.  It confirms various suspicions I have always had, such as the fact that the magnetic parameters of nickel at RF are unpredictable so it is safe to assume they are very bad, and that nickel should never be used for RF coils.  I have been a consultant to a lot of companies doing high power RF and I am always on the look out for nickel plating.  The clients will tell me how great nickel is for various reasons.  But I have to tell gently tell them it is a non starter for RF.  No the customer is not always right.
> 
> The company I currently work for has air core inductors custom made using ordinary tinned copper bus wire.  I have measured the inductance of these coils and it compares very favorably with the predicted Q of our coils if bare copper wire were used.  (See Medhurst's 1947 paper on the subject).  Tin does have considerably lower conductivity vs copper, but at least it is non-magnetic.  Also, the thickness of the tin plating is so thin that it is negligible in terms of skin depth.  Even when it is old wire that is no longer shiny bright, it still doesn't ding the Q.
> At least for indoor work, tinned copper for my money is the way to go.
> 
> 73
> Rick N6RK
> 
> On 8/2/2025 10:15 AM, Wes wrote:
>> Actually, I know it's a bad idea, see top of page 151 of the following.
>>
>> https://www.dropbox.com/scl/fi/w5u2m0qvuwjs0i3vlwdsf/Plating.pdf?
>> rlkey=n99dlrd2joh5548drtczvtsks&st=grysczj9&dl=0
>>
>> On 8/1/2025 2:29 PM, Wes wrote:
>>> I would need some convincing that nickle plating is a good idea.
>>>
>>> Wes  N7WS
>>>
>>
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