[Lowfer] A couple of crude Antenna Matching techniques - will either one work
Zack Widup
w9sz.zack at gmail.com
Fri Dec 1 11:07:38 EST 2017
At my last job I had to measure SWR of some special antennas operating
in the 100 to 800 kHz range. I made an SWR bridge using ferrite
toroids that covered that frequency range. It was accurate and worked
quite well. Unfortunately I don't work for that company any more and
the SWR bridge is far away. I can always make another one.
73, Zack W9SZ
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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