Calculating loading coil for ⅝-over-¼ colinear GP (⅞)

[BlackBeard]

After a lot of thinking I decided to try the ⅝-over-¼ colinear GP - as described here http://on5au.be/content/a10/vhf/cc.html by Cebik - also known as the "curly colinear". I know that Albert H mentioned the ⅞-GP often as well as the need for a loading coil (I guess it's the same antenna as the one Cebik discussed). I'd be more than happy to get the dimensions for your ⅞-GP - especially the loading coil.

For 435MHz, Cebik mentions an inductance of 0.29µH. Keeping in mind the relation between capacitance, inductance and frequency, I calculated a value of 4.9µH for 105.4 MHz. I have no clue though, if this is right.

Also, I'd be glad if anyone could explain to me, how to calculate the proper inductance of the loading coil.

Do I need a second coil, or can I connect the lower end of the ¼λ-piece directly to the center/+?

What about common mode/sheath waves? As usual with GP-type antennas, I'd implement a simple rf choke by forming a coil using the coax (like the one used in the flowerpot).

[EFR]

http://gal-ana.de/basicmm/en/

Thats quite simple tool to mess around with diffrent desings

[BlackBeard]

http://gal-ana.de/basicmm/en/

Thats quite simple tool to mess around with diffrent desings

Thanks, I'll give it a try. I also tried EZNEC 7 with NEC 3 but wasn't successful. Adding loads/coils isn't too easy.

[BlackBeard]

Here are my results. The VSWR (1.07) seems to be okay with a 3uH loading coil. Nonetheless, the gain values are way off. When comparing the gain over real ground to a simple dipole, the dipole has more gain! Cebik's calculations resulted in a gain of about 7.55 dBi (=5.4 dBd). Had to zip the antenna's .maa file, because the forum software doesn't accept any other file format.

[EFR]

On phone now, so cannot open that fule, but on image it looks like you have an little kink on that red current line?

[BlackBeard]

I ditched the 5/8 over 1/4 for now - in favor of a simple 0.64λ as described here by Albert: viewtopic.php?t=1599
It's shorter and still gives a decent - realistic - gain of about 4dBd. Radiator is 1.82m and ground planes are 0.912m. GPs seem a little long to me - but maybe that's because of the radiator's extended length.

Nonetheless, the gain values shown by MMANA-GAL are weird. It also shows me about 6.81 dBi gain over real ground for a simple - well matched - vertically polarized dipole. Which is utter gobshite of course

[BlackBeard]

Okay, correction. I changed the ground planes' size to ¼λ and the VSWR also got better - close to 1. My fault

[EFR]

Yoou havv something wrong in your grund settings.

http://gal-ana.de/basicmm/en/

Code: Select all

Ga represents the absolute gain.

Gh is derived simply by substituting 2.15dB from Ga. Gh is not displayed if the ground option is included.

Make sinple dipol, adjuts settings, there is video in Youtube how to.

After you have settings right, your dipole shows right gain of about 2.1-2.2dBi

Im drunk, laying on couch.

[Albert H]

I still really like the 0.64-wave - It's pretty easy to get dead right!

[rigmo]

D3izKOU.jpeg

I still really like the 0.64-wave - It's pretty easy to get dead right!

.64 wavelength "secret". Can anyone attest to its effectiveness over traditional 5/8 (.625) wavelength?

[Albert H]

0.64 tends to be easier to match (it's less critical). The losses from poor VSWR can be significant. There's no point in using your expensively generated RF to heat up the coax!

[BlackBeard]

0.64 tends to be easier to match (it's less critical). The losses from poor VSWR can be significant. There's no point in using your expensively generated RF to heat up the coax!

Thanks Albert I don't wanna bother you too much but is the coil hard to get right? Is it critical for the radiation pattern or do the coil and it's inductance simply need to be tweaked until the SWR is right?