Inductances needed to shorten dipole

[perky]

Does anyone have any info on what inductance values are needed to shorten a 868MHz dipole and where they should be placed? I'm about 20mm short in space in a box for either arm, it would be nice if I had a long thin FR4 PCB with a UFL connector at the feedpoint and inductors placed somewhere (possibly close to the feedpoint) in series with the arms but have no feel for their values or what the best position for them might be. Thanks!
Mark.

[captcha]

Not sure on values, but since the highest current is at the feedpoint you don't want them there. Real world loaded verticals have an optimum placement of loading coils around 2/3 to 3/4 away from the feed point.

[perky]

I found this link that might be quite useful, apparently putting the inductors closer to the ends is more efficient:
http://www.k7mem.com/Electronic_Notebook/antennas/shortant.html
Mark.

[emjay]

@perky,

One approach avoids using discrete inductors provided you have some "width" available for the dipole. The trick is to meander the arms. Since you only need a small shrink factor, a 0.75 meander looks close and does not upset the feed impedance too much. A good reference is here. Note that if your construction is on FR4, there is a shrink factor for free anyway due to a "velocity factor" from not being in free air.

[john4444]

Hi Perky,

info on what inductance values are needed to shorten a 868MHz dipole

I'm making a couple of assumptions that if incorrect means that this solution will not apply.
1 This is a small diamter copper wire antenna, say... 22-AWG wire.
2 You have some free space around the antenna.
3 SWR is not a critical parameter
IMHO the easy way to shorten the overall length of a 868-MHz antenna
(which will start out at roughly 6.5" oa that you want to shorten to 5.8")
is to try putting a couple of turns midway along each arm.
If you do 1/8"-dia loops, then you only need 1 to 2 turns on each arm.
A closely spaced turn will shorten the arm length by about 3/8" but you will want to
stretch the turns so that they are widely spaced. So... two turns stretched out to 3/8".

This should be easy to try. If you still have the range that you want, then you are good to go.
If it does not work like you want, just straighten the wire and you are back to where you started.
The actual inductance values for such an antenna will be tiny.
The reference you listed is for antennas in the ham bands, much lower frequiencies than 868-MHz.

Good Luck, John

[perky]

Thanks John. I was looking at putting a PCB in the box for ease of manufacture, however I found this link about bent dipoles which implies there's not much penalty in gain or radiation pattern if they are bent at the ends. So I'm tempted now to put a very small PCB just with a UFL connector and two through-holes for wire arms and bend the ends at 90 degrees. That'll also give me the opportunity to experiment by cutting down the lengths when in the actual box to get to resonance:
https://www.nonstopsystems.com/radio/pdf-ant/article-bnt-dipole.pdf

@emjay: Thanks for the link. Meandering is an option but I'm thinking of doing the above now as it would be easier to tune.

Mark.

[john4444]

Perky,
I'm not visualizing your PCB, sorry.
But, we are very lucky that the RFM69 is a very tolerant radio.
You have lots of options that will work... well enough.

Your new link is very interesting.
If one were trying to get very good SWR & impedance matching
then, bending the ends could help to achieve those goals.

Just keep other conductors as far from the antenna as possible.
Good Luck, John