[Elecraft] Sherwood's receiver performance table updated

Tue Apr 26 17:21:49 EDT 2016

Jim,

The problem with your question is that the answer really is "it all 
depends".
If the feedline from the antenna is short and relatively low loss, then 
the ATU  in the shack is fine.
Or if the feedline is very low loss (say open wire line, well supported 
and away from conductors), then the length of the feedline does not 
matter much at all, and again the ATU in the shack is the more 
convenient answer.

OTOH, if the feedline is coax, and the length is for instance 150 feet, 
then you would want to consider something at the antenna. Whether that 
is a remote ATU, or some type of fixed matching section to bring the 
antenna feedpoint impedance down to a range of 30 to 150 ohms would be 
in order.  With the fixed matching section (antenna loading), you would 
also want to have an ATU in the shack.

I have generalized on the antenna type - but the answer remains the same 
no matter what the antenna type.  Look at a beam with a matching section 
at the driven element - that is one example of a fixed matching section, 
it will not likely be flat SWR over the entire band and an ATU in the 
shack can keep the PA transistors working into a 50 ohm load as you QSY.

Another example of a fixed matching section is a matching inductor (and 
possibly capacitor) that is used on many of the popular 43 foot 
verticals - but you still need an ATU in the shack.

One solution for a vertical antenna is to feed it with low loss parallel 
feedline.  Before you scream 'heresy' because the vertical is 
unbalanced, there is nothing wrong with using a parallel feedline for a 
vertical, the RF will figure it out - the radials are merely the "other 
half" of the monopole.  If your feedline run is long, consider using 
open wire line if you can, or ladderline as a close second.  Yes, you 
need a current choke at the vertical antenna, and that takes care of the 
'balance' situation - use a 1:1 choke, you are simply choking common 
mode current and not trying to match the impedance of the feedline.

73,
Don W3FPR

73,
Don W3FPR

On 4/26/2016 4:09 PM, Jim Rodenkirch wrote:
> Don: requiring a wide ranging ATU is half of the question, Don!! What is
> truly "significant" is this - many hams, new or otherwise, fail to
> understand whether they really need an ATU in the shack or do they really,
> REALLY need an ATU out at the antenna. For me, THAT's the mystifying piece
> of the "ATU question" that always shows up.....
>
> 71.5/72 de Jim Rodenkirch K9JWV
>
>
> Don Wilhelm wrote
>> Wunder,
>>
>> The only reason I can understand is that they can get away by stating
>> that they have an internal ATU.  Sadly many hams (particularly newer
>> hams) gloss over the significance of the need for a wide range ATU.
>> They don't discover that they need an external tuner for their
>> "multiband" antenna until after they buy the transceiver and discover
>> that it is inadequate.
>>
>> 73,
>> Don W3FPR
>>
>> On 4/25/2016 7:25 PM, Walter Underwood wrote:
>>>> On Apr 25, 2016, at 4:08 PM, Wayne Burdick <
>> n6kr@
>> > wrote:
>>>>     - *Wide-range ATU (> 10:1 at 100 W; > 20:1 at low power) and two
>>>> antenna jacks
>>> I am really mystified about why transceivers include a 3:1 range ATU. It
>>> adds $150-200 to the end cost to get an external ATU. It might add $50-75
>>> to make the internal ATU wide-range. It is especially odd for an
>>> entry-level rig, where people are likely to be using a low-slung dipole.
>>> OK, it is very strange for the IC-7851, too.
>>>
>>>
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