Confession?

[DaveInDenver]

I need to read your previous post a few times to understand better.....

It's EM trickery. An antenna resonant at 1/4λ at some frequency/wavelength will have the same characteristic impedance (e.g. 37Ω) at every odd multiple. So your ~19" long antenna is a 1/4λ at 146MHz means it'll be 3/4λ at 3 x 146 = 438 MHz, where it'll also have 37Ω impedance but the radiation pattern will be different. It's also resonant at even multiples, e.g. 1/2λ, at 292MHz but this isn't a voltage null but rather a current null so the impedance is extremely high (several kΩ) and requires a matching network to 50Ω. Υou may not remember me or perhaps a EE prof talking about voltage and current phases and how that impacts impedance in reactive EM fields so I'll just leave it to the student to investigate if you wish.

 

[DaveInDenver]

I need to read your previous post a few times to understand better.

I have a Flexiwhip on the truck roof for a 2m APRS radio so I swept it twice, once across VHF and a second time across UHF. It's about 19.25" tall measured from the roof.

First, the intended band. Markers are 146.00 MHz where it measures 75.76Ω and a SWR of 1.60. It's clearly long since minimum SWR is below the amateur band, 142 MHz or so. You can see the bandwidth of a 1/4λ antenna here. The SWR is set to 0.5/division and frequency is 700 KHz/div, so crossed 2:1 at about 137.4 MHz on the left side and is still below 2:1 at 150 MHz. Therefore at least 150 -137.4 = 12.6 MHz of bandwidth.

This is what it looks like to the radio when I sweep across UHF. Marker is 450.00 MHz, at the top of our 70cm allocation. It shows 33.07Ω and an SWR of 1.52. This is the same 19.25" tall Flexiwhip. It's actually a flat SWR from 426 MHz up to 456 MHz, so it's reasonably wide band here, too. Goes 2:1 at 423 MHz and 457.5 MHz, so at least 34.5 MHz of bandwidth.

 

[rover67]

Well, I just got done tuning the 5/8 Wave antenna, it started way long. Crazy to see what trimming 1/4" does..... My radio (Kenwood TM-V71) seems to like a lower SWR..... and I was WAY off with the antenna out of the box.... or at least with the replacement whip thrown on there. I couldn't help myself since the data is fun to plot and I liked Dave's visual, so I plotted my results so I could look at it and to help guide me.

I could go a bit shorter but I'm just going to leave it as is. It seems good enough and favors the middle to lower part of the band which I'm OK with

 

[rover67]

I needed to replace the AC compressor so I took the opportunity to shrink the drier. It’s a four seasons part number 33592 for a 89-2014 911 and 98-2001 Chevy tracker of all things. Also for a 94-95 rx-7.

 

[Corbet]

I think the Camary drier from same model year fits and is shorter. My ARB covers mine so I have not tested this rumor.

 

[simps80]

@rover67
What ac compressor did you buy I have to do this on mine

 

[rover67]

I got a Denso compressor from Rock Auto, $189. Wasn’t bad I thought. The 1997 Camry drier is 7” long vs the one I used which was 6.5” long. I chose the shorter one since it tucked in better.