The coil in the base of a "Silver Rod"

[ghost123uk]

Does anyone know what the design / spec of the coil in the base of a basic 1/2 wave "silver rod" type antenna is please?

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Reason is, I wish to build a "Silver Rod" type antenna, but using a thin, tapered glass fibre (not carbon fibre) vertical with a copper wire up the centre.
I wish it to be as "discrete" as possible.
I know I "could" do this with a 49:1 Balun, but I seem to remember the coil (balun?) in a "Silver Rod" is very simple.
Please note, I don't want to build a TL2T or anything similar

[ch25]

ant.jpg

[Stronty]

If you search my posts I’m sure i uploaded a picture of one I had in the early 80’s.

Update:

viewtopic.php?t=57574&hilit=Silver+rod&start=45

Last page of that thread. Not sure about exact length and I’m not at home just now so can’t measure.

[ghost123uk]

Thanks to both Lemmy (again) and Stronty

I will now get myself a tapered 18 foot FG pole and give this idea a whirl.

[InTheClouds]

My suggestion is that a fibre glass flag/fishing rod will let damp in through the joints over time so use insulated wire to protect the copper. During testing/tuning I would initially attach the wire (just tape it) to the outside of the pole for easy access to add or snip wire off.

Once you have calculated the radiating element length including a velocity factor add about an extra 10-20cms just to be sure, it is easier
to snip wire off during tuning than solder it back on. Antennas do not always quite tune as expected in real world locations. Do not forget to gorilla glue blob the exposed open end of your insulated copper wire or at least cover it well with insulating tape (though that tends to come off over many thermal cycles)

You may also wish to consider adding a 0.05 wavelength (minimum recommended) to the ground side. It may not be necessary but recall silver rod type antennas are usually attached to a metal pole which provides a random length (depending on the users pole length) counterpoise. (0.05 Lambda will be about 52cms of wire). The antenna has a confusing attribute for many in that the radiating element is DC shorted... i.e. continuity is shown between the radiating element and the bracket (ground side). However RF is AC not DC and so the RF's AC sees a different situation than a short to ground via the inductive coil matching it.

Whilst it may confuse a little these are worth a read, but in short just make a CP 0.05 wavelength long and you will be fine. It may well work without one at all but a CP may assist with tuning and obtaining a lower SWR reading. Myself I use a 0.05 CP and do extremely well with DX.

Explanation why 0.05 wavelength CP is useful:

https://www.aa5tb.com/efha.html

More about this antenna type, various opinions.

https://ham.stackexchange.com/questions ... ound/12393

Sounds like fun to me...we would all like to see it once complete : )

[davev8]

There are 2 types of coil at the bottom of a silver rod .. 1 is just a loading coil the other which is more common is a tapped inductor
How the loading coil works is the physical length of the antenna is 1/2 wave or 5/8 wave but the coil electrically lengthens the antenna to 3/4 wave... A 3/4 wave works the same as a 1/4 wave and doesn't need a matching device..a car antenna like a 9-foot whip is 1/4 wave so don't need anything to match it ,,its just adjusted to a 1/4 wavelength ..now say a 4 foot 6 inch car antenna is 1/8 wave and the coil in it electrically lengthens the antenna to 1/4 wave so no matching device is then needed
the 5/8 wave Sirio tornado base antenna works like this the coil electrically lengthens it to 3/4 wave.....reading this you may now realize that mobile antenna manufacturers that say things like 5/8 or 7/8 wave on the box are fibbing as they can only be 1/4 wave or we would have use a ATU or something to match it
A tapped inductor works differently ..One end of the coil attaches to the radiator the other end connects to the ground to which the coax braid is conected to ..now the center of the coax connects or taps into the coil somewhere along its length..moving the position of this tap will adjust the SWR...now determining the size of the coil can be tricky for the DIYer as the spacing, and the diameter and the number of windings affect the value of the coil....however for a 5/8 wave it can be simple if you only have 1 winding in the coil and a set length,, with 1 winding there is no problem of keeping the correct spacing between coils as there is only 1 and with a set length the diameter will always be the same it looks just like a ring ..you may have seen antennas with a ring at the bottom
I have never made a 1/2 wave but have numerous 5/8 waves ..i make the ring out of car copper brake pipe as it bends without kinking
the length is 6 feet one end goes to the radiator the other end to the ground, and a wire from the center of the coax goes to the middle
of the ring and is adjusted one way or the other to adjust SWR ..you need 1/4 wave radials on a 5/8 wave i made them out of stout wire and used them for part of the guy ropes ...here is a photo with dimensions for a DIY 5/8 wave ...you can make a 1/2 wave but may have to experiment with the length of tubbing for the ring

[InTheClouds]

Sorry OP this will go a bit "hammy" but that is how we can all learn if of any interest.

I have made 1/2 and 5/8 waves DIY from scratch. This might be worth a bit more discussion. I have heard that a 5/8 wave is extended to 3/4 wave electrically (i.e. the actual physical length is not 3/4 wave) to get a closer match - along with the radials (elevated resonant or random length ground mount) which also changes impedance a little.(I think it is stated as such even in the FULL ham manual that a 5/8 is electrically extended to be 3/4 wave - multiples of 1/4 waves potentially providing "resonance", depending on precise definition of the word.

Although I have seen the definition of a resonant antenna as being.. when capacitive reactance and inductive reactance is zero without requiring coils or LC matches... like in a 1/4 wave GPA or a 1/2 wave dipole that typically needs no such arrangements. (notwithstanding a 1:1 balun that many will use)

I ask openly ask... is there a difference between a coil to match impedance (with a tap or specific exact coil size, so it does not need a tap) and a coil to increase electrical radiator length ? (which will also change impedance of the wire, by virtue of a coil being attached).

They seem rather like the same thing. Although worth noting that we do not usually call a 49:1 transformer a loading coil. And we do not usually call a loading coil an impedance transformer.

I have for example never seen a 1/2 wave end fed with reference to making it 3/4 wave long using the 49:1 (for a single band). Just that the usual 49:1 toroid/transformer and capacitor is an LC device that matches the high impedance of the 1/2 wave wire/alloy pole to approx 50Ohms. (i.e. an impedance transformer)

I have seen hams throw the term resonant around a lot. I am not entirely sure if it means any antenna can become resonant (with matching of some kind) Or if the only truly technically resonant antennas are those that naturally match to 50 Ohms approx (i.e. the capacitive/inductive reactance cancel at the feed point with no matching device required - easy example 1/4 wave ground plane with 45 degree sloping radials.)

This is more of a question than a definitive answer.

(For the OP/CBers let's stick with 1/2 wave silver rods - as you can see there is a tapped coil in the bottom and this does a very similar task as what hams call a 49:1 transformer (multiple enameled wire turns on a ferrite ring - a very basic AC transformer). One difference is most hams use the 1/2 wave in a different way - instead of using on 1 band like a CB user would i.e. 11m band - they use it on multiple bands - so for basic example they may use 20m of wire and operate that antenna on 10m and 20m bands with an acceptable SWR for each band - albeit usually horizontally unless portable)

[InTheClouds]

A few things that confused me in your post Davev8 and details and how things are written are important in understanding anything.

"now say a 4 foot 6 inch car antenna is 1/8 wave and the coil in it electrically lengthens the antenna to 1/4 wave so no matching device is then needed "

The coil is the matching device.

"the 5/8 wave Sirio tornado base antenna works like this the coil electrically lengthens it to 3/4 wave.....reading this you may now realize that mobile antenna manufacturers that say things like 5/8 or 7/8 wave on the box are fibbing as they can only be 1/4 wave or we would have use a ATU or something to match it
A tapped inductor works differently ."

But the Sirio 5/8 wave is a tapped inductor - as is the matching arrangement on my own 5/8 wave - a tapped coil. I think you are making a distinction between a single turn like your big dummys guide image, though it is still tapped according to your drawing based image. (just there is one loop and no coil)

Attached is GPE 58 image you can see the solder joint where the tap is.