JI1FGX/DU9 Amateur Radio Diary, Mindanao, Philippines, IOTA OC-130.
English. Japanese.



Yesterday's access.
Introduction. [Lily Diary.】
A diary of life in Mindanao.

25/09/13  3.5MHz SWR increased
25/09/12 3.5MHz Zepp antenna adjustment
25/09/10  Geostationary satellite QO-100
25/09/09  Preparing for the satellite station
25/09/08  DU9JJY satellite station
25/09/07  14MHz late at night
25/09/03  Mitsubishi Strada Pickup Truck
 Oil Change
25/08/18 WSJT-X Automatic operation Ver0.1.0
25/08/18  JTDX Autonomous operation Program Ver0.6.3
25/08/10  Software List
25/07/26 AutoCWType_Ver1.5.8
25/07/20  Create one month's worth of ADIF files from JTDX Ver 2.6.5
25/07/19  Japan's Radio Wave Usage Tax
25/07/13  IARU HF Contest
25/07/10 Software updates
25/07/08 DXV500ZS linear amplifier repair
25/07/07 CQ Machine Program
25/06/21 LoTW system upgrade
25/06/18 RG8 for 3.5MHz arrived
25/06/15  Time setting Ver0.5
25/06/14 Drone Habsan ZINO arrives
25/06/12 Philippine Independence Day
25/06/11 Algeria 7X2RF QSL card.
25/06/10 Translation API Program
25/06/09 DXCC150 award certificate arrived
25/06/08 AutoCWType_Ver1.4.1
25/06/07 Introducing the Shack
25/06/06 A portrait drawn by ChatGPT.
25/05/26 Windows full-width/half-width switching.
25/05/17 OK2ZAW BCD to 16 converter.
25/05/16 For 3.5MHz Zepp antenna.
Stepping motors
25/05/15 3.5MHz stepping motor design.
25/05/14 3.5MHz tuning coil installed.
25/05/11 3.5MHz antenna installation completed.
25/05/03 JTDX Autonomous Driving Program Ver0.4.3.
25/04/26 Drone Habsan ZINO.
25/04/25 Time setting Ver0.3.
25/04/24 AutoCWType_Ver1.3.
25/04/23 Preparing for FTDX3000 LCD repair.
25/04/22 Installation plan for 3.5MHz Zepp antenna.
25/04/21 AutoCWType_Ver1.1.
25/04/20 10,14MHz antenna installation completed.
25/04/19 JTDX Autonomous Driving Program Ver0.4.1.
25/04/19 14MHz antenna pole installed.
25/04/18 ThinkPad X390 repair completed.
25/04/17 Search is now possible.
25/04/15 Preparing the 10MHz dipole.
25/04/12 Hexbeam Part8.
25/04/06 My ThinkPad X390 is broken.
25/04/05. Time setting program.
25/03/31 JTDX Autonomous Driving Program.
25/03/30 Automatic log sending from CWType to Hamlog.
25/03/21 Hexbeam Part 7.
25/03/20 FreeDV Part 3 First QSO.
25/03/18 FreeDV Part 2 QSO in the shack.
25/03/16 Hexbeam Part 6.
25/03/15 Install FreeDV Part1.
25/03/09 Hexbeam Part5.
25/03/07 Hexbeam Part 4.
25/03/05 28MHz antenna modification
25/03/01 FTDX3000 is broken!
25/02/28 Pileup at 50MHz FT8.
25/02/28 DXV500ZS linear amplifier repair.
25/02/27 Tower pipe rebuilding plan.
25/02/26 Direction to Japan from Ozamiz.
25/02/26 28MHz is not available.
25/02/23 DXV500ZS linear amplifier repair.
25/02/22 Hexbeam Part 3
25/02/21 Hexbeam Part 2
25/02/18 28MHz antenna construction.
25/02/17 18,24MHz antenna height construction.
25/02/16 Z26NS Cosovo.
25/02/14 21MHz FT8
25/02/12 7,21MHz antenna repair.
25/02/08 Operating CW at 21MHz.
25/02/05 DXV500ZS linear amplifier failure.
25/01/19 Input Director.
25/01/14 Temporary license renewal.
3.5MHz SWR increased (2025/09/13)
When operating the FT8 for a long period of time with a 3.5MHz Zeppelin antenna, the SWR rose to 2.0.
SWR2 is the value before adding the ferrite core.

The SWR was 1.0 for the first 7 seconds or so of the FT8 transmission,
then gradually increased until it reached SWR 2.0 just before the end of the transmission.
If you stop transmitting for about 5 minutes and then start transmitting again, the SWR value will be normal for a while.
 
When I touched the FT240-43 ferrite core, it was too hot to touch.
It seems to be causing 'Saturation (magnetic saturation)'

To be more specific:
When a large RF current flows through a ferrite core, the magnetic flux density reaches its limit and the magnetic permeability drops sharply.
As the temperature rises further, some materials approach the Curie temperature and lose their magnetism.
In actual operation, this appears as the 'Heat generation reduces the blocking impedance, making the choke ineffective.' phenomenon.



Now (provisional operation)
Directly below the feed point: The main component is an air-core choke (RG8, 30 cm diameter x 20 turns = approximately 20 m of coaxial cable).
  • 4 m downstream: Insert a #43 choke in line.
  • If it heats up, remove it = it's safer to use the air core alone.
  • The aim is 'The air core receives the large current at the power supply point, reducing the burden on #43'.

When #31 arrives (permanent operation)
  • Recommended configuration (prioritizing stability) Directly below the power supply point = #31 stack Two FT240-31s stacked (=stack) + 12-15 turns of RG58
  • Two is a big improvement. Three gives you even more room (ideally two to three).
  • RG58 is OK (no differential power flow/short length). Just be careful about heat resistance.
  • 4–5 m downstream = air-core choke
  • Use with a 30 cm diameter x 16-18 turns (RG8 recommended)
  • Using two stages in series increases the combined Zcm, dispersing heat and effectively suppressing the gradual rise in SWR.

'If you have #31, do you not need an air core?'
  • End-powered (Zepp) makes it easier for common mode current to flow through the coaxial cable, so '#31 + two-stage hollow core' is more stable than #31 alone.
  • If you really want to have one level, '#31 Stack Only' will work, but considering the margin (heat resistance and seasonal fluctuations), it is recommended to use both.
  • When using them together, you can reduce the air core 20T to 16-18T to make handling easier.

Specific setup example (assuming up to 600 W)
  • Directly below the feed point: FT240-31 x 2 (preferably x 3) + RG58 12-15T
  • 4–5 m downstream: air core (30 cm×16–18T, RG8)
  • The switcher side, 20 m away, can be added as an optional third choke (to prevent noise backflow). It is not the main component.


Production notes
  • Do not wind turns too closely (align each turn) / Do not wind close to or parallel to metal surfaces or elements
  • Waterproof and heat dissipating outdoors (be careful of heat buildup when completely sealed)
  • Test: FT8 15 s transmission followed by 15-30 s pause, several sets, checking SWR trends and core temperature.
  • Bobbin diameter: 30 cm (= radius 15 cm)
  • Circumference of one turn: π × 0.30 m ≒ 0.942 m
  • Number of turns: 20 Winding spacing (sparse winding): 10 mm = 1 cm
  • Bobbin length (height direction) 20 turns x 1 cm = 20 cm
September 12, 2025.   September 13, 2025.