Looking for small (1' tall?) antenna design

**ags0000** · 10-29-2019, 01:19 PM

I "built" a 1/4 wave antenna (from a hub and some #12 AWG solid wire) that works great. It lives inside my megatree, suspended from the top with a simple string. I can continue to use it as-is without concern.

I was thinking that it may actually be as effective for radio transmission to take the antenna out from the shielding of the pixel strand "Faraday cage" and place a smaller one on top of the tree, outside the pixels (in front of the star). For aesthetics, I'd want this antenna to be no more than about a foot tall.

Is there some design that I could make myself? I suspect not, as some kind of loading coil would be needed for an antenna that is less than 1/4 wavelength. I do have a small "ground plane" (about one square foot) at the tree top, and I was thinking I might even just mount a small "rubber ducky" antenna.

I'm not an RF engineer/designer (or ARRL member). Can anyone with more knowledge in this area chime in? Thanks.

**ukewarrior** · 10-29-2019, 02:21 PM

I agree that the smallest antenna you can build is a 1/4 wave ground plane.
I've used one for years. If you look in the radio waves part of the wiki, you should find some DIY plans.
If not, PM me with your email as I'm pretty sure I have a plan I can send you.

**ags0000** · 10-29-2019, 03:01 PM

Originally Posted by ukewarrior

I agree that the smallest antenna you can build is a 1/4 wave ground plane.
I've used one for years. If you look in the radio waves part of the wiki, you should find some DIY plans.
If not, PM me with your email as I'm pretty sure I have a plan I can send you.

A plan for the 1/4 wave, or the "normal-mode helix antenna" (rubber ducky)? I've been using the 1/4 wave I built for years - it works great. I'm not looking for increased performance, rather I have plenty of performance and will sacrifice some for a smaller antenna (if DIY build is possible).

Thanks.

**BuzzKill** · 10-29-2019, 04:49 PM

Originally Posted by ags0000

Is there some design that I could make myself? I suspect not, as some kind of loading coil would be needed for an antenna that is less than 1/4 wavelength. I do have a small "ground plane" (about one square foot) at the tree top, and I was thinking I might even just mount a small "rubber ducky" antenna.

A question that comes to mind is will stepping down to a rubber ducky degrade the signal more than having had it where it was as a proper dipole? If you take regular copper wire and work harden it, (https://hackaday.com/2018/07/18/harl...-antenna-hack/ ) I can't see where a thin copper wire dipole would be that obvious on the front or back of your tree, especially at night. If you wanted or could install elsewhere, there are some interesting plans for FM broadcast J-Pole, and SlimJim, antenna designs you can make from copper pipe. But they will be certainly more obtrusive than a plain dipole design.

**ags0000** · 10-29-2019, 04:53 PM

Yes - just hanging a vertical length of wire, fed in the center, to act as a dipole would be easier to handle than the 1/4 wave ground plane I have now. Not sure how radiation pattern will be affected by proximity to the center support post (2-1/2" sched. 40 pipe).

I wonder if I could find a "rubber ducky" tuned for low VHF (91 MHz in my case) and just give it a try. Shouldn't harm my xmtr if tuned properly.

**aknflyer** · 10-30-2019, 02:09 AM

A rubber ducky antenna would work fine, it is just a full antenna length wrapped in a coil to save space. If just using a wire antenna, you could use a folded V or a double folded V, if it is too tall for your needs. All of these have some range drawbacks, but if range is not an issue feel free to experiment. The range we should be using is in the hundreds of feet. It pains me to see users pushing 8Watts. Mine is a 100mw transmitter in a half sunken basement that still reaches too far. Working on attenuating that down.
40 yr telecom tech.

**ags0000** · 10-30-2019, 04:52 PM

Agreed - I only need to reach out (with good musical quality) as far as one can see, which in this case is a couple of blocks. I'm not interested in bouncing signals off the moon. I've never heard of the folded/double-folded V antenna, will use Google on that.

I think one problem with using a rubber ducky is finding the right one for ~92MHz. Most of what I've found available for purchase is targeting handheld UHF transceivers. From the research I did, they are very lossy outside a narrow bandwidth.

I found a few sites discussing the theory, but the diameter of the coil, the coil spacing and pitch/angle are pretty complex to get right, so DIY didn't seem too promising. Not that I'm against trial-and-error, but I'd hate to get it wrong, end up with a very bad SWR, and fly my transmitter output.

**aknflyer** · 10-31-2019, 01:43 AM

Antenna theory is a very intricate subject. Probably only engineers will ever get it. You did'nt mention your transmitter type and power output. Me, I just use a cheap dsp pll 100mw transmitter, I keep a spare and never worry about experimenting, since they cost less than $10. If your really worried about reflected power, we use a device called a circulator, it shunts all reflected power to a dummy load, will never damage the transmitter. Not sure about the cost for a hobbiest.

**ags0000** · 11-01-2019, 12:37 PM

I actually have EE background - but not recent experience in RF/antenna design. I'm using a (non-disposable) EDS transmitter, at the minimum (100mW, I believe) output.

I'd be happy to just use an off-the-shelf ducky antenna; problem is they all appear to be tuned for (high) VHF/UHF use, for handheld transceivers. My research indicates that these antennae have limited output and small bandwidth, so those standard models may be a problem.

To avoid this, I was thinking of just wrapping some solid #12 AWG to make my own. The four variables in the ducky-style antenna design are coil diameter, coil separation, wrap pitch and length. I haven't yet found information that would allow me to calculate reasonable values for each, or a template of what someone else has done.

There are (were) some folks @ DIYC that seem to have extensive RF design knowledge. One in particular offered a wealth of knowledge, but apparently became uninvolved after some debates about this domain of knowledge. I was hoping there might be someone still active that might offer some advice from their own technical/academic background.

**pakeonoahu** · 11-02-2019, 02:01 AM

how about this? cut a wire with the length(ft)=234/Frequency, wind your coil, get a cheap VSWR bridge https://www.ebay.com/itm/RF-bridge-0...-/332052527822, connect your transmitter, antenna coil, and a cheap $20 SDR radio to the bridge, start trimming/cutting off your wire from the coil little at a time and look for the lowest dip/frequency(the resonance frequency of your antenna) on your SDR graph, that would be your correct length of the tuned coil antenna.