Antenna Match


Pete McQuain
 

Hello all,
I recently completed a "classic" pi-network match box consisting of 2 14-365pf air variable caps and an approximately 20uH coil with 12 taps (36 turns on a 1" pvc former, tapped every 3 turns going to a 12-pole rotary switch). I'm going to be using this with my 5w20 kit and my QrpLabs QCXPlus 40 with a few different antennas. I've been testing it with my Xiegu G90 and a 35' end-fed "random wire", as well as measuring with a VNA. I can get a perfect match on 40m, but have a hard time getting it matched on 20m.
I've done some more research on different network types - T, L, etc. and I'm thinking perhaps I should reconfigure it to be a T-network, any thoughts on that?
Also, I'm still wrapping my head around the theory involved in matching networks and the reactance/resistance, etc. and am not clear if I can take the C's in parallel with the L to calculate the tuning range or if that's me over-simplifying the RF black magic going on in there :)
Any help or advice would be appreciated.
Thanks!
Pete K4UGG


 


On 6/3/21 11:12 AM, Pete McQuain via groups.io wrote:
Hello all,
I recently completed a "classic" pi-network match box consisting of 2 14-365pf air variable caps and an approximately 20uH coil with 12 taps (36 turns on a 1" pvc former, tapped every 3 turns going to a 12-pole rotary switch). I'm going to be using this with my 5w20 kit and my QrpLabs QCXPlus 40 with a few different antennas. I've been testing it with my Xiegu G90 and a 35' end-fed "random wire", as well as measuring with a VNA. I can get a perfect match on 40m, but have a hard time getting it matched on 20m.
What is your antenna load?
I've done some more research on different network types - T, L, etc. and I'm thinking perhaps I should reconfigure it to be a T-network, any thoughts on that?
T network mimics a high pass filter while PI is a low pass filter; also PI uses higher value components than T
Also, I'm still wrapping my head around the theory involved in matching networks and the reactance/resistance, etc. and am not clear if I can take the C's in parallel with the L to calculate the tuning range or if that's me over-simplifying the RF black magic going on in there :)

Antenna matching only required one capacitor and one inductor;

 My favorite is where the inductor is between the transmitter and antenna and the capacitor is switched to either across the transmitter or antenna

-Diz, W8DIZ

Any help or advice would be appreciated.
Thanks!
Pete K4UGG


Pete McQuain
 

Thanks for the info. So if I understand correctly, you would put the inductor in series and switch the capacitance to ground before or after the inductor depending on Zout being higher or lower than Zin (50ohm)? I am about to build another, smaller version using polyvaricons and a T130-2, but I want to be able
to switch each cap in or out, getting the best of hi-Z and low-Z L networks and a T network. Probably overkill, but it might lend itself well to experimenting with antenna configurations. 

This is probably too big of a question for this format, but, how does a matching network acting like a hi or low pass filter work with impedance? Is it cutting off/passing the frequencies themselves or the reactance at those frequencies?

thanks again!
pete K4UGG


 


On 6/5/21 2:56 PM, Pete McQuain via groups.io wrote:

Thanks for the info. So if I understand correctly, you would put the inductor in series and switch the capacitance to ground before or after the inductor depending on Zout being higher or lower than Zin (50ohm)?

YES!

I am about to build another, smaller version using polyvaricons and a T130-2, but I want to be able
to switch each cap in or out, getting the best of hi-Z and low-Z L networks and a T network. Probably overkill, but it might lend itself well to experimenting with antenna configurations. 

This is probably too big of a question for this format, but, how does a matching network acting like a hi or low pass filter work with impedance? Is it cutting off/passing the frequencies themselves or the reactance at those frequencies?

thanks again!
pete K4UGG