scaling antennas

[davev8]

on 11 meters I have often made a 5/8 wave based on the dimensions in the screen grab you can see it has a ring made from 6 feet copper tube ....now If I made a 20 meter 5/8 wave I can scale up the radiator that off the top of my head is 41 feet ish and radials at 16 or 17 feet ....but Will the length of the ring still be 6 feet ? Or is there something better to match it ? thanks

 

[binrat]

I've made a couple, for 10 and 2 metres. I scaled to the frequency which seemed to work.

 

[davev8]

I've made a couple, for 10 and 2 metres. I scaled to the frequency which seemed to work.

so you are saying i need to scale the matching ring as well ,,,,,i did not know as they are both 5/8th wave end feed ..i think maybe the impedance may be the same for both at the feed point ... which if it was the ring i would think could be the same size for both ...

 

[binrat]

I scaled everything down. For 2 meter it was several attempts at the ring before I was happy.

 

[The DB]

so you are saying i need to scale the matching ring as well ,,,,,i did not know as they are both 5/8th wave end feed ..i think maybe the impedance may be the same for both at the feed point ... which if it was the ring i would think could be the same size for both ...

The antenna doesn't see the inductance itself, it sees the reactance the inductor adds to the antenna. To get the same reactance on an antenna scaled to a different frequency requires a different inductor. Luckily, all you have to do is scale the ring with the antenna to compensate for the needed change in reactance.

So, in easy terms, yes you need to scale the ring with the antenna if you want a match.


The DB

 

[Christopher17]

going to try something like this, building my first 5/8 wave , assuming there a specific place to tap the ring and would most likely be frequency specific as to the tap location.?

 

[binrat]

I would wrap the wire around the ring so adjustment can be made, then solidify it once tuned. I use copper tubing and soldered once tuned.

 

[davev8]

The antenna doesn't see the inductance itself, it sees the reactance the inductor adds to the antenna. To get the same reactance on an antenna scaled to a different frequency requires a different inductor. Luckily, all you have to do is scale the ring with the antenna to compensate for the needed change in reactance.

So, in easy terms, yes you need to scale the ring with the antenna if you want a match.


The DB

Thanks DB

 

[davev8]

going to try something like this, building my first 5/8 wave , assuming there a specific place to tap the ring and would most likely be frequency specific as to the tap location.?

yes just make a sliding connection and make permanents when corect ...mine have always been temporary antennas made out of scrap ..a fiberglass hammer shaft is good to replace the wood in the photo ....if i remember i think 1/3 from the antenni end of the ring may a good starting point ...but you soon notice if you are sliding it the wrong way ...