[Elecraft] HI CUR on 40

Sat Oct 31 23:21:47 EDT 2020

If there is ever any question about where to place ferrites, I’ll say in general, put the ferrites at both ends of the coax (unless its a really short run). 

Some may argue that that is overkill. While there are always exceptional circumstances, I would in general respectfully disagree with them if they did. While the benefits of placing them at the feed point are well documented, I have found that replicating the ferrites at the other end of the coax generally reduces noise in the receiver, making it possible to hear and thus work more stations… ;-) 

It would also be part of the solution to rf in the shack, provided that the ferrites provide enough choking impedance. 5000 ohms at the frequencies in question would likely do, but referring to K9YC’s papers for the specifics is advised.

Ferrites at both ends, along with proper bounding and grounding of the whole station a la K9YC has always worked well for me. 

73,
David - N5DCH

> On Oct 31, 2020, at 7:07 PM, David Olean <k1whs at metrocast.net> wrote:
> 
> 
> Hi Mike,
> 
> I would put the ferrite at the end of the coax before it goes into the rig if there were rf coming down the coax. Common mode energy can and will get on the outside of the coax. I am not an expert on this, but I would expect that any problem that would cause a transmitter to misbehave would  involve strong levels most likely encountered near the antenna where the fields could be high.   Of all the folks that have used that 5 element six meter yagi, (me included) No one has ever had a stray RF problem. Running the coax perpendicular to the elements and tied down to the boom is all that is needed to keep things quiet.  I am at a loss to explain any other outcome.
> 
> I suppose there could be a fault in the balun area, but I would think that the SWR would climb well above 1:1 in that case.
> 
> Dave K1WHS
> 
> On 10/31/2020 11:13 AM, M Cresap wrote:
>> Dave
>> 
>> So where is the optimum place to place the ferrites for this antenna if you have a problem like Alan described?
>> 
>> Does the near field extend roughly to half the stacking distance of that antenna (i.e. 6 feet or so), making that the best place for the ferrites?
>> 
>> Does a mostly vertical coax passing through the near field of a horizontally polarized antenna (i.e. an antenna side mounted to a tower) need ferrites?
>> 
>> Do you need to decouple ALL of the rest of your coaxes and control lines at the lower near field edge of that antenna (or at some other point)? I have heard that argument (decouple everything) made to improve performance of inverted L antennas on 160 meters located near a tower.
>> 
>> 73, Mike, W3IP
>> 
>> On Saturday, October 31, 2020, 10:04:48 AM EDT, David Olean <k1whs at metrocast.net> wrote:
>> 
>> 
>> Hello Alan
>> 
>> That was my antenna design. It has a teflon coax 1/2 wave balun that
>> kills RF on the shield side of the coax, but there is not much you can
>> do about near field energy getting picked up by the outside of the coax
>> due to radiation.  It is best to tape the coax down snugly along the
>> boom and masting etc.  Still, it does not hurt to add more RF chokes to
>> any antenna! It is pretty easy for RF energy to get impressed on the
>> outside of the coax in any typical installation.  The balun at the
>> antenna only keeps the energy from the coax center conductor from going
>> to the outside of the coax at the feedpoint.
>> 
>> The way I check for decoupling effectiveness is to match the antenna
>> really well, and then run your hand along the outside of the feedline
>> near the feed. While looking at the screen of a network analyzer, you
>> can see little perturbations in the return loss 20 or 30 dB down if
>> there is RF leaking to the outside of the feedline. This is a
>> qualitative test and you can estimate the bandwidth of the decoupling
>> network. You will see larger return loss change amounts away from the
>> resonant frequency of the decoupling element.  This won't apply to
>> ferrites, as they can be broadband, but with a quarter wave choke, or
>> similar, you can definitely see the effect.
>> 
>> 73
>> 
>> Dave K1WHS
>> 
>> On 10/31/2020 5:39 AM, N3ALN wrote:
>> 
>> > Hum, I was getting this HI CUR error only with my Directive Systems 6 Meter
>> > antenna (DS50-5) that has a 1:1 SWR I hope they know antenna design? Maybe
>> > not? The toroids fixed my issue. I just love how some of the forum or so
>> > judgmental, it is a only a freaking hobby, chill.
>> >
>> >
>> >
>> > --
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