Skip to main content
Folks have been aware of this round-tripping for a long time. :/

I want an ICOM IC-705 for our car. :( #AmateurRadio #HamRadio
#hamradio #amateurradio

It's doing what it's supposed to do, but I need to add a variable capacitor to compensate for the added inductive reactance.
Decent impedance match @ 50 MHz (X = 32.61 + j3.152Ω); SWR = 1.544)
Dipole resonant frequency @ 52 MHz (X = 15.55 + j26.44Ω); requires compensation for the inductive ractance added by the autotransformer

Deep Thoughts — Litterbox Comics

Let's see if this autotransformer works. #AmateurRadio #HamRadio

Neil E. Hodges

2025-11-16 05:28:45


All that work to build a parallel coax transformer and it still doesn't cover the whole 6m band. :(

I don't know why, but this LMR-240-75 was way harder to strip and break out into terminals than normal 50Ω LMR-240 is. :/

#AmateurRadio #HamRadio

Parallel coax transformer to step the impedance down by half, for 6m
NanoVNA: SWR of 1.048 @ 51.7 MHz, but the SWR dip doesn't cover the entire 50-54 MHz band

#hamradio #amateurradio

All that work to build a parallel coax transformer and it still doesn't cover the whole 6m band. :(

I don't know why, but this LMR-240-75 was way harder to strip and break out into terminals than normal 50Ω LMR-240 is. :/

#AmateurRadio #HamRadio

Parallel coax transformer to step the impedance down by half, for 6m
NanoVNA: SWR of 1.048 @ 51.7 MHz, but the SWR dip doesn't cover the entire 50-54 MHz band
#hamradio #amateurradio

Busy busy. :3 #OpenTTD
#openttd

The more advanced order stuff in #OpenTTD sure is nice. :3
OpenTTD screenshot: Orders for a train featuring conditional orders and refitting to available cargo
OpenTTD: Cargo for the same train right before hitting its first major unloading point
#openttd

👍 Resonant frequency
👎 Radiation resistance

#AmateurRadio #HamRadio

#hamradio #amateurradio

Parallel coax impedance transformers work so well! :D

Every church needs its own oilfield! :P #OpenTTD
#openttd

Vance really is acting like he's this. :P

#github #microsoft #azure
#github

That looks a lot like a docked PowerBook Duo. (Image source)

Anyone recognize the face on the right? That's not how Dead Hand normally looks. (Image source) #zelda
#zelda

Relevant.
Microsoft is a corporation that turns "Microsoft CEO claims 30% of its new code is written by AI" into "Windows 11 Update KB5063878 Repeatedly Causing SSD Failures"
Microsoft is a corporation that turns "Microsoft CEO claims 30% of its new code is written by AI" into "Windows 11 Update breaks localhost, prompting mass uninstall workaround"
Amazon is a corporation that turns "Amazon Allegedly Replaced 40% of AWS DevOps Workers With AI Days Before Crash" into "Major AWS outage took down Fortnite, Alexa, Snapchat, and more"

Did CGA predict the trans flag colors? :3c (Image source)

What sort of abomination have I created? :P #AmateurRadio #HamRadio
Weird "H" antenna
NanoVNA showing an SWR well below 2 for the entire 70cm band and more!
#hamradio #amateurradio

Guess it's time for a new map. :( #OpenTTD
#openttd

Finally back on 70cm. The dipole as-is can only cover the upper ~14 MHz of 70cm, but I'm going to see about getting some even smaller brass tubing so I can extend the legs even further. 🤔

And if that doesn't work, I have plenty of variable capacitors in my shack. 👍

#AmateurRadio #HamRadio

How much is that dipole in the window?
NanoVNA @ shortest length: 44.13-j6.785Ω @ 446 MHz (SWR of 1.278)
NanoVNA @ longest length: 39.59+j10.49Ω @ 433.8 MHz (SWR of 1.392)
#hamradio #amateurradio

👀 What's this? #Meshtastic #LoRa
#meshtastic #lora

Finally started getting serious about interchanges in #OpenTTD . :3
#openttd

What if life-expectancy grows substantially?

Don't think we have to worry about that. :/ (Source)

Wow, alcohol is worse than heroin and cocaine here. :O

Table from the 2010 ISCD study ranking various drugs (legal and illegal) based on statements by drug-harm experts. Methadone was found to be the 12th overall most dangerous drug.

Now that is more like it. 👀

#AmateurRadio #HamRadio

70cm dipole at its current length
NanoVNA: SWR of 1.357 @ 435 Mhz (36.85 + j548mΩ)
#hamradio #amateurradio

Mr. Bean would be a more competent leader than Donald Trump. :P

Kind of want to make a halo antenna. :3 #AmateurRadio #HamRadio
#hamradio #amateurradio

Princess Mommy

Another thing that makes me think it's the coax is that I was seeing an "extra" resonance when the dipole's self-resonance was at a lower frequency. #AmateurRadio #HamRadio
Self-resonance of the dipole itself: 74.97-j6.25Ω @ 420.0 MHz (SWR of 1.52)
Coax resonance?: 66.94+j5.595Ω @ 468.0 MHz (SWR of 1.359)
#hamradio #amateurradio

After switching to brass legs for my 70cm dipole for strength, I started trimming and filing them down to get something working.

  • At ~132mm measured from the coax, it was resonating well at 424.5 MHz with an impedance of 44.24+j1.987 Ω and an SWR of 1.138.
  • At ~107mm measured from the coax, the impedance at 436.0 MHz was approximately halved with an impedance of 23.41+j7.66 Ω and an SWR of 2.200.

Between the feed point and opposing UHF connector, the coax is about 114mm long, which is very close to two wavelengths.

(%i1) 1.14 / (%c / 436E6 * 0.83);
(%o1) 1.9975262006020027

I'm getting the sneaking suspicion that the coax is doing unwanted impedance transformation, but I'm not sure. What do folks think?

#AmateurRadio #HamRadio

Dipole legs @ 132mm
NanoVNA: 44.24+j1.987 Ω @ 424.5 MHz (SWR of 1.138)
Dipole legs @ 107mm
23.41+j7.66 Ω @ 436.0 MHz (SWR of 2.200)
#hamradio #amateurradio

On a train. :D

Ended up getting longer legs (in aluminum because the hardware store was out of the copper tubing I like) and switching to a proper common mode choke winding pattern…and it just worked. It covers the entire 70cm band with SWR≤2! :O

I do have some more aluminum tubing that fits snugly inside these dipole legs, so I can make the legs telescoping to reduce the resonant frequency.

#AmateurRadio #HamRadio

Full view
The T200-2 choke balun in its enclosure
NanoVNA showing an SWR≤2 for the entire 70cm band. Resonant frequency is 442 MHz with an SWR of 1.285
#hamradio #amateurradio

Huh. Wasn't expecting a T200-2 core to work as a choke for 70cm (with a longer antenna than shown here). :P #AmateurRadio #HamRadio
Choke: 5×2 turns of 16 gauge wire on a T200-2 core
SWR under 1.05 at 465 MHz. Bandwidth is 37 MHz.
#hamradio #amateurradio

⇧