A Socially Distance Field Day

By: John EI1761

DTG :1000-1600hrs 03/01/2021
Location: IO63lf or Rathangan, Co. Kildare

Before Christmas 2020 we as a club managed to do a remote field day experiment with NVIS from my village location into the valleys and hilltops around Skerries, see other write up.

So it was time for me to step up and put a few certain chapters of the IRTS study guide and YouTube into reality. The agenda was to build an 80 meter dipole and tune it by reducing the leg lengths in a controlled manner until it was resonant in the middle of the band I want to use. So yeah that's the long and short of it... what's missing is the phone calls, WhatsApp video group chats and our friendly Amazon which didn't close for Christmas.

With a dipole center borrowed from Tom (EI5IEB), a 33-meter spool of speaker wire and taking over the kitchen on St. Stephen's day I got to work, an annoying thing about the majority of online sites that explain dipole is they convert the frequencies to Meter band plans yet give the measurements in feet.

So in an ideal world, the dipole center would be about 6.5 meters off the ground and the length of each leg would be 19.4 meters.... but some websites say it could be anywhere between 13.15 meters and 21.29 meters depending on the gauge of wire based on an imperial inch thickness scale... Christ on a bike is it no wonder I get distracted trying to study for the HAREC. So back to a lovely frosted Sunday morning and off to the Men’s Shed. I wanted a nice large area that isn't in the public eye to set up without trying to answer questions from the public. 

Steps I Took

  1. Drink some coffee
  2. Uncoil the dipole legs - give out to yourself for newbie mistake of coiling wrong way.
  3. Drink another mouthful of coffee.
  4. Connect aerial to nanovna to get a baseline.
  5. Scratch head for a while.
  6. Disconnect VNA and connect Radio to see if you can pick up anything. listen to two lads rag chewing one in Floirda the other in Finland.
  7. Celebrate that aerial picks up traffic with a mouthful of coffee.
  8. Jump on to ELARC WhatsApp group to update the lads.
  9. Share screenshots of VNA plots.
  10. Adjust VNA to display 80m band edges to make tuning easier. Cheers Dom (EI5IAB)
  11. First adjustment was to reduce each leg by 1 meter,
  12. A better SWR reading - adjust some more by reducing by a further 50cm
  13. A lot better reading now, advised by Frank (EI8HIB) to reduce by the magical figure of 220mm?
  14. Drink another mouthful of coffee and another newbie error of leaving the VNA turned on while making adjustments in cold weather just runs the battery down very fast.
  15. Quick 5min charge in the car to get the final SWR reading of the day.
  16. Disassemble of kit.

Kit used.

Bodum coffee cup 

Speaker Wire From Amazon
Dipole Centre From Long Communications with custom strain relief

Yaesu FT-891

Lesson learned

  • Write the steps down from YouTube video rather than just following along particular for the equipment you can't use at kitchen table aka VNA on aerial.
  • Coffee in Bodum cup is still burning hot after an hour in the cold.
  • Coil wires for deployment not for convenience.
  • Ensure your checklist includes batteries for accessories and not just ones for the radio,

The narrative below complied from content of the Clubs WhatsApp Group Chat and videos with John during the antenna build has been compiled by Frank

John’s first sweep indicated a resonant freq. of 3.436MHz, the calculated length of an antenna at that freq. is 20.81m. The Freq required is 3.7MHz which has a calculated length of 19.31m.

Difference is 20.81m – 19.31m = 1.5m. However, at this stage one doesn’t know what the real Velocity Factor (Vf) is. So best to play safe and reduce the electrical length by something conservative, say 1m.

So, advised John to reduce the leg length by 1m. This gave him a resonant Freq of 3.579MHz. So heading in the right direction.

Calculated theoretical electrical length of 3.579MHz as 19.98m, the targeted length is 19.32m. So new calculation is 19.98m – 19.31 = 0.67m, which lines up approx. with the first calculation.

John reduced the antenna leg length by 0.5m, which gave him a new resonant Freq of 3.644MHz, (Nearly there). Following the same routine, calculated electrical length of 3.644MHz is 19.62m, the targeted length is 19.31m (3.7MHz). So new calculation is 19.62m – 19.31 = 0.31m. I advised John to reduce the length by 220mm, (not 310mm) as it’s good practice to approach the target Freq carefully. Much easier to take off a bit more wire than to add it back on.

The final adjustment gave a resonant. Freq. of 3.678MHz, with an SWR of 1.08/1. With an acceptable SWR across the Phone part of the band.

So generally speaking, be conservative with the removal / or folding back of wire on the antenna. Measure twice, cut once. When your finally happy with the SWR, it’s a good idea to remove most of the folded back wire. This will slightly change the resonant Freq again, and the effective diameter of the antenna, so bandwidth will change. So you will need to leave enough wire to Unfold more and possibly redo the same tuning exercise.

Also bear in mind that height above the ground will affect the resonant frequency, so if this changes dramatically, it will be necessary to redo a sweep to confirm the SWR at the required frequency.

Photographs and text for context by John

The first reading from the NanoVNA showing sweep from 80m to 20m. With Dipole legs just over 20 m each leg.

Second reading showing sweep of 80m 2.5MHz to 3.8MHz. Dip is centered on 3.436MHz

Third reading reduced each leg by a meter, sweep is still 2.5MHz to 3.8MHz. but Dip is centered on 3.579MHz

Fourth sweep reduced each leg by a futher .5 meter, sweep is still 2.5MHz to 3.8MHz. but Dip is centered on 3.644MHz

Final sweep after brief interruption, reduced each leg by the magical 220mm, and adjusted sweep to 3.6MHz to 3.8Mhz dip is centered on 3.678MHz

For this one I also took SWR readings. 

3.6MHz is 1.57

Dip at 3.678MHz is 1.08

3.8MHz is 1.83

Coffee drank so time to head off. But happy out I got that far. Now just waiting for my Mast and the courage to add on 40m and 20m fan dipole legs to it, and one final thing, passing the licence exam when it comes round so I can play with these toys.


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