If you are following the instructions included, that is for 10 m. start with 22.5 feet from where the coax screws in to the tip and fine tune from there.
-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
New spt500 gamma
- Thread starter Van Lifeson
- Start date
True, we was trying everything. The second picture of the same was the second SPT-500 after we took the first one back. Both are NIB.
As we are hams, We tried all the way out, and all the way in... same, both new antennas from DX
Please read my original comment on this thread. 28.4 Mhz is MY goal.
Did you try shorter than 19'? My records show 10 meter goes up up to 28.305 mhz. When I get my house finished I will start on the Ham journey.
10 Meters covers 28.0MHz to 29.7MHz, he wants the antenna to be centered at 28.4MHz.
28.305MHz is where the phone part of 10 Meters starts.
Did you measure the overall length of the antenna at 33-34 MHz, it had to be shorter than 19'. It would just be interesting to know......
Why not remove the red part (the shunt inductor);
Then using a VNA, adjust the length of the antenna until it crosses this line (the constant conductance circle that goes to 50Ω, the only place a shunt inductor can bring the impedance to 50Ω);
then, once on that conductance circle somehwere, use that value to calculate the shunt inductor value needed to go up to 50Ω and make a different hairpin that works.
To make a hairpin inductor of a specific inductance, two things must be known, the characteristic impedance of the hairpin (as a transmission line if the end was not shorted but kept going forever) and the electrical length from the open end to the shorted end (in degrees). Xl = Zo * tan (length).
To know what Zo of the hairpin is, you need the spacing between the conductors and their diameter. Zo = 276 * log(2s/d).
Edit: I still find myself wondering a few things.
Then using a VNA, adjust the length of the antenna until it crosses this line (the constant conductance circle that goes to 50Ω, the only place a shunt inductor can bring the impedance to 50Ω);
then, once on that conductance circle somehwere, use that value to calculate the shunt inductor value needed to go up to 50Ω and make a different hairpin that works.
To make a hairpin inductor of a specific inductance, two things must be known, the characteristic impedance of the hairpin (as a transmission line if the end was not shorted but kept going forever) and the electrical length from the open end to the shorted end (in degrees). Xl = Zo * tan (length).
To know what Zo of the hairpin is, you need the spacing between the conductors and their diameter. Zo = 276 * log(2s/d).
Edit: I still find myself wondering a few things.
- why have the GP above the feed?
- why have that green ground section connected to the red hairpin and not just bring the hairpin to the bracket?
- is it worth more if it looks more complicated?
Last edited:
At 28MHz, 3/4 wave is 26', so to have it in the capacitive region (necessary for shunt inductor matching), it needs to be between 1/2λ and 3/4λ. How much depends on where the impedance crosses that conductance circle (without the hairpin). The end result is something close to 5/8λ so thats what they call it.
I'd play around between 22 and 24 feet without the hairpin in place and see if you can get the impedance (at the feedpoint, not at the vna input) to cross that constant conductance circle. Once there, stop adjusting the length and either calculate the new inductance needed or post your vna readings.
I'd play around between 22 and 24 feet without the hairpin in place and see if you can get the impedance (at the feedpoint, not at the vna input) to cross that constant conductance circle. Once there, stop adjusting the length and either calculate the new inductance needed or post your vna readings.
Last edited:
It's not gamma. It's a beta, or a hairpin match. I have one here,, they claim the Hy-Gain design was subject to breaking, so they changed it.
Nothing beats the original gamma on my original 70's penetrator. The new cheaper crap that you have to bend yourself is a joke.
This is not the SP-500 of years ago, they should have left it alone.
They all worked fine back then and were simple to tune, but they "fixed" it...........
I ran a old one for years before I switched to a I-10K and it weathered storms just fine, never broke.
The material they are building them out of is soft, the "redesigned" match was really bent out of shape out of the box.
MFJ has struck out again.
73
Jeff
They all worked fine back then and were simple to tune, but they "fixed" it...........
I ran a old one for years before I switched to a I-10K and it weathered storms just fine, never broke.
The material they are building them out of is soft, the "redesigned" match was really bent out of shape out of the box.
MFJ has struck out again.
73
Jeff