The Shop > Electronics & IC Programing
Amatuer Radio Operator Members
SKIPRAT:
Well i am guilty of being an amateur radio operator mainly 2 Metres and 70cm bands to those over the pond 145mhz and 440mhz and as for the HF bands i have antennas and equipment for the 10 Metre band and wait in anticipation for the solar cycle to pick up a bit more so i can get some worldwide DX in the bag.
Cheers Paul 73 DE G6FOW
75Plus:
I have accumulated the necessary parts for the impedance matcher. I will put them all together and explain the operation tomorrow. Stay tuned....
Joe
75Plus:
--- Quote from: SKIPRAT on April 28, 2011, 06:11:34 PM ---Well i am guilty of being an amateur radio operator mainly 2 Metres and 70cm bands to those over the pond 145mhz and 440mhz and as for the HF bands i have antennas and equipment for the 10 Metre band and wait in anticipation for the solar cycle to pick up a bit more so i can get some worldwide DX in the bag.
Cheers Paul 73 DE G6FOW
--- End quote ---
Hi Paul, We use the same terms, i.e. 2 Meters and 70 cm on this side of the pond also. We just change the spelling a bit. :beer: :beer:
Joe
75Plus:
Back to the project. As we hams know, in getting the maximum efficiency from an antenna one has to deal with two elements, tuning and matching. Tuning is adjusting the length of an antenna for the band or portion there of on which you plan to operate. An antenna can be tuned properly, by obtaining the lowest possible standing wave(SWR), but still not be matched. The second element is matching the impedance of the antenna. When one "takes it on the road", operating mobile, there is always trade offs. There are limits to what can be safely and efficiently used in this kind of operating. Even the vehicle, to some extent, governs the type of antenna that can be used.
The most common, and least expensive mobile antennas, have the lower half made from fiberglass and has copper wire wound around it. The number of turns is determined by the band which it was made for. The top half is a stainless steel whip. The antenna is tuned by adjusting the length of the whip. Usually this can get you an SWR of just under 2 to 1 the goal is 1 to 1. Getting there is what this project is about. The impedance still needs to be matched.
About 20 years ago I picked up a box of WWII surplus fixed frequency crystals mounted in FT243 enclosures. I planned to use the enclosure to house an impedance matching capacitor. I needed a way to quickly change this capacitor when I made a band change. This is what I came up with.
This is the drawing that I was working from. Several of the holes are countersunk but the free version CAD program I use doesn't provide for them. The pictures help in this regard.
Here are all the parts ready to be assembled. The brass bit in the center of the top row is the grounding strap for the lower contact. The sockets are made from 10-32 screws that have been bored 1/2" deep with a 3/32" drill. The end of the screw that is still in the nylon block has been drilled and tapped for a 4-40 screw that secures the grounding strap. I used stainless screws but I have made these using brass. Just need to stay away from anything that can rust. The lower right part is 1 1/2 X 3/4" stainless, drilled and tapped 3/8-24 from both ends.
Here you can see the capacitor residing in the enclosure. This one is 330pf which matches 40 meter antenna.
To determine the value needed for a given antenna I stuck a 365pf variable capacitor in an aluminum enclosure and fitted a plug so it could be plugged in to the adapter. Once the antenna is tuned I just remove it and, using a capacitance meter, measure the value and then fit the nearest standard capacitor.
This is how it looks when in use. Seeing a crystal, plugged into an antenna, draws a lot of attention at Ham gatherings. Everyone is trying to figure out what it does and how it does it.
I moved in for a closer look.
Joe
DavidA:
Skiprat,
...and wait in anticipation for the solar cycle to pick up a bit more so i can get some worldwide DX in the bag....
Acording to Spaceweather.com the sun is fair crackling. Any good contacts ?
Dave.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version