Silver Rod Question(s)

This is the place to discuss any general CB radio related topics. Getting started, installations, operation etc.
Post Reply
User avatar
Buick Mackane
Moderator
Moderator
Posts: 9560
Joined: 21 Aug 2012, 17:30
Location: A citizen of legoland

Silver Rod Question(s)

Post by Buick Mackane »

Antenna theory was never my strong point but i understand the basics. I'm trying to figure out how silver rods work, Yeah too much time on my hands :lol: So here's what's bugging me,

A halfwave dipole on 27mhz is roughly eighteen feet long, Nine feet either side. Now a halfwave silver rod is eighteen feet long, Shouldn't the main element only be nine feet long with the ground planes accounting for the other nine ?

The way im seeing it is like a lope sided dipole with one element being eighteen feet long and the other element being only nine feet long. There must be some kind of transformer converting the impedence

I'm probably overlooking the obvious but it is 2 am :lol:

So how do these magic wands work ?
I Am the great cornholio are you threatening me ?
TrunkY
Registered New User
Registered New User
Posts: 1
Joined: 25 Oct 2021, 15:44

Re: Silver Rod Question(s)

Post by TrunkY »

It's presumably a half wave vertical with a 49:1 UnUn at the base.
User avatar
Buick Mackane
Moderator
Moderator
Posts: 9560
Joined: 21 Aug 2012, 17:30
Location: A citizen of legoland

Re: Silver Rod Question(s)

Post by Buick Mackane »

TrunkY wrote: 16 Feb 2022, 09:36 It's presumably a half wave vertical with a 49:1 UnUn at the base.
Cheers would make sense. Kinda like a vertical long wire.
Last edited by Buick Mackane on 16 Feb 2022, 10:15, edited 1 time in total.
Reason: Additional
I Am the great cornholio are you threatening me ?
User avatar
DX-Digger
Top Poster
Top Poster
Posts: 1076
Joined: 27 Jul 2009, 22:01
Call Sign: TM205
Location: ... - --- -.- . --- -. - .-. . -. -

Re: Silver Rod Question(s)

Post by DX-Digger »

There is a coil in the base to match the high Impedence and then a tap point which represents the 50Ohm impedence point


Image
:thumbup:
ʎɐqǝ uo pɹɐoqʎǝʞ ɐ ʎnq ı ǝɯıʇ ʇsɐן ǝɥʇ sı sıɥʇ
User avatar
Buick Mackane
Moderator
Moderator
Posts: 9560
Joined: 21 Aug 2012, 17:30
Location: A citizen of legoland

Re: Silver Rod Question(s)

Post by Buick Mackane »

DX-Digger wrote: 16 Feb 2022, 13:46 There is a coil in the base to match the high Impedence and then a tap point which represents the 50Ohm impedence point


Image
:thumbup:
Ideal DX Digger :thumbup: I knew there had to be some impedance matching device in it,

Much appreciated
I Am the great cornholio are you threatening me ?
User avatar
NightProwler
Veteran
Veteran
Posts: 3397
Joined: 08 Feb 2012, 22:31
Call Sign: 163TM186
Location: South Wales

Re: Silver Rod Question(s)

Post by NightProwler »

I opened up an old silver rod years ago and the coil was a thing bit of wire wrapped around a small cardboard tube. Quality.
Shorter of breath and one day closer to death
User avatar
Buick Mackane
Moderator
Moderator
Posts: 9560
Joined: 21 Aug 2012, 17:30
Location: A citizen of legoland

Re: Silver Rod Question(s)

Post by Buick Mackane »

NightProwler wrote: 16 Feb 2022, 14:17 I opened up an old silver rod years ago and the coil was a thing bit of wire wrapped around a small cardboard tube. Quality.
They just don't make them like that anymore richy :lol:
I Am the great cornholio are you threatening me ?
26OD004
Regular
Regular
Posts: 10
Joined: 01 Nov 2017, 21:35
Call Sign: 26OD004

Re: Silver Rod Question(s)

Post by 26OD004 »

Buick Mackane wrote: 16 Feb 2022, 02:12 Antenna theory was never my strong point but i understand the basics.

So how do these magic wands work ?
I'll tell you how I understand how they work, and hopefully it might help...

I think you have to consider why we use a half wave end fed like an antron or silver rod, it is pure convenience, a half wave dipole is so easy to put up horizontally, but not so easy vertically, which is what you want for CB. You can feed a half wave dipole at any point along its length and it will work just the same, the only thing that changes is the impedance. We use a half wave dipole and feed it in the middle because at the middle the impedance is close to 50 ohm, which matches to the 50 ohm coax which matches to the 50 ohm radio.

If you want to feed the dipole at the end, so we can put it at the top of a pole we have to feed it at the end. The impedance at the end is in the 1000's of ohms, so a bad match for the coax. The coil is there because at one end of the coil there is 1000's of ohms, the other end is connected to ground so 0 ohms, stands to reason that somewhere along the coil is 50 ohms, you just have to find it, and that's where you connect your coax.

Both antenna's should work the same, only difference is the loss in the matching coil whatever that may be.
User avatar
Buick Mackane
Moderator
Moderator
Posts: 9560
Joined: 21 Aug 2012, 17:30
Location: A citizen of legoland

Re: Silver Rod Question(s)

Post by Buick Mackane »

26OD004 wrote: 18 Feb 2022, 21:58
Buick Mackane wrote: 16 Feb 2022, 02:12 Antenna theory was never my strong point but i understand the basics.

So how do these magic wands work ?
I'll tell you how I understand how they work, and hopefully it might help...

I think you have to consider why we use a half wave end fed like an antron or silver rod, it is pure convenience, a half wave dipole is so easy to put up horizontally, but not so easy vertically, which is what you want for CB. You can feed a half wave dipole at any point along its length and it will work just the same, the only thing that changes is the impedance. We use a half wave dipole and feed it in the middle because at the middle the impedance is close to 50 ohm, which matches to the 50 ohm coax which matches to the 50 ohm radio.

If you want to feed the dipole at the end, so we can put it at the top of a pole we have to feed it at the end. The impedance at the end is in the 1000's of ohms, so a bad match for the coax. The coil is there because at one end of the coil there is 1000's of ohms, the other end is connected to ground so 0 ohms, stands to reason that somewhere along the coil is 50 ohms, you just have to find it, and that's where you connect your coax.

Both antenna's should work the same, only difference is the loss in the matching coil whatever that may be.
Good explanation 26OD004

I Get it now, Really need to make more of an effort to brush up on my antenna knowledge,


Thanks for that :thumbup:
I Am the great cornholio are you threatening me ?
26OD004
Regular
Regular
Posts: 10
Joined: 01 Nov 2017, 21:35
Call Sign: 26OD004

Re: Silver Rod Question(s)

Post by 26OD004 »

Buick Mackane wrote: 18 Feb 2022, 22:10
26OD004 wrote: 18 Feb 2022, 21:58
Buick Mackane wrote: 16 Feb 2022, 02:12 Antenna theory was never my strong point but i understand the basics.

So how do these magic wands work ?
I'll tell you how I understand how they work, and hopefully it might help...

I think you have to consider why we use a half wave end fed like an antron or silver rod, it is pure convenience, a half wave dipole is so easy to put up horizontally, but not so easy vertically, which is what you want for CB. You can feed a half wave dipole at any point along its length and it will work just the same, the only thing that changes is the impedance. We use a half wave dipole and feed it in the middle because at the middle the impedance is close to 50 ohm, which matches to the 50 ohm coax which matches to the 50 ohm radio.

If you want to feed the dipole at the end, so we can put it at the top of a pole we have to feed it at the end. The impedance at the end is in the 1000's of ohms, so a bad match for the coax. The coil is there because at one end of the coil there is 1000's of ohms, the other end is connected to ground so 0 ohms, stands to reason that somewhere along the coil is 50 ohms, you just have to find it, and that's where you connect your coax.

Both antenna's should work the same, only difference is the loss in the matching coil whatever that may be.
Good explanation 26OD004

I Get it now, Really need to make more of an effort to brush up on my antenna knowledge,


Thanks for that :thumbup:
Happy to help, antenna theory still confuses me now, but I try :geek:
Post Reply