[Lowfer] A couple of crude Antenna Matching techniques - will either one work

[John Langridge]

>1) Andy may be suprised how well his HF SWR bridge will work on 630m. The Pout readings may be off, but the null for lowest SWR will be there even if incorrect in magnitude.

This depends on how the sampling is accomplished.  I had an MFJ HF
cross needle meter a few years ago that used ferrite sampling  which,
based on discoloration of the core after use at MF, suggests that it
got hot and probably exceeded the Curie point as it doesn't work
anywhere now.

If you have one of the units that have closely space sampling bars
tied to diodes and either dual meters or a switch to change forward
and directed power, you could be ok but verify what you have first.  I
didn't really care about burning up the MFJ meter anyway.  oh, btw,
before that meter burned up, it read 2:1 SWR on MF no matter what the
load.  On HF it was verified to be working properly into a good load
until it finally failed.

73!

John

On Fri, Dec 1, 2017 at 9:54 AM, Brian, WA1ZMS <wa1zms at att.net> wrote:
> My 2 cents on this.....
>
> 1) Andy may be suprised how well his HF SWR bridge will work on 630m. The Pout readings may be off, but the null for lowest SWR will be there even if incorrect in magnitude.
>
> 2) Tuning under TX power could be an issue for some TXs based on the class of operation the PA uses: Class A, B, C, D, E. Switch modes amps can act strange under active load pull. Any amp can too though.
>
> 3) An ideal meathod for live tuning under TX power (for me) has been to place a 6 to 10dB power attenuator in the PA output path. The PA will run with a "reasonsble" load while you go outside and tune away for max antenna current. The RF power and current at the antenna will be much lower, but also less chance of getting RF burns as you tune and poke around. An attenuator might be cheaper (homebrew) than an antenna analyzer.
>
> 4) I've done all the above for 2200m & 630m myself.
>
> -Brian, WA1ZMS
>
>> On Dec 1, 2017, at 10:14 AM, John Bruce McCreath <weazle at hurontel.on.ca> wrote:
>>
>>
>>
>> Hi Andy….can you clarify a few things for me.  Is the bottom end of your coil grounded, is the capacitor
>> in parallel with the coil, are you tapping the feed line inner conductor near the bottom end of the loading
>> coil, and is its shield connected to the bottom of the coil/cap?  If that’s the case, you should be able to
>> bring it all to resonance with the meters you have.  I wouldn’t put an RF ammeter at the top end of the
>> loading coil, as it could really throw things off because of the high voltage there.  The antenna current
>> should be the same, regardless of whether it’s measured at the “cold” or “hot” ends of the coil.  Do you
>> have a means of measuring the transmitter final current?
>>
>> By comparison, here is how I resonated EAR’s antenna.  The loading coil has a built-in variometer for
>> adjusting resonance.  I put a current-transformer type RF ammeter on the ground end of the coil and
>> watched for a peak while I rotated the inner coil of the variometer.  To match the final to the loading
>> coil/variometer, I used a three turn link coil wound over the very bottom turns and moved it around
>> while watching for a dip in the final current.
>>
>> 73, J.B., VE3EAR
>>
>> LowFER Beacon "EAR"
>> 188.830 kHz. QRSS30
>> EN93dr
>>
>>
>>
>>
>>
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