The QRPer Board

Brian - K3ES wrote: ↑Tue Jan 31, 2023 10:35 am I finished my build this month, and used the antenna for Winter Field Day... Er... used Winter Field Day to test the antenna. Particulars are:

- 9:1 unun auto-transformer was built on a FT50-43 core, wrapped with 9 turns of tri-filar (3 twisted strands) 24 AWG magnet wire.
- I connected a 144 ft radiating wire, with the objective to enable tuning for CW on the 10m through 160m bands.
- I used a three 17 ft wires for a counterpoise.
- LDG Z-11Pro II tuner was successful matching 10m, 15m, 20m, 40m, 80m, and 160m bands with 1.5:1 or better SWR.

I am pleased with the results, and am working on a more complete field report, so more to follow. Thanks to all who helped with tips and advice.

73 de Brian - K3ES

Hi there, Brian, nice job and happy to hear that your "random" is working well, sure 144ft isn't exactly a "portable" length, but if that's ok for you, fine with me notice though that a shorter length covering the 80m band, with a good counterpoise, could allow to "tune" down to 160m although being short er than 1/4 lambda the antenna won't be so efficient there

Anyhow, if you want to try building an ATU, have a look at this design

https://pa0fri.home.xs4all.nl/ATU/FRIma ... tcheng.htm

it can be built for QRPp using two polyvaricons or one may use a couple of regular air var caps to build a version supporting QRO, the only limitation of that ATU is the fact that it doesn't tune down to 160m, or better said, it could do so by changing the coil, but then it will just tune "up" to the 30m band, not a big issue but worth mentioning, on the other hand, that tuner has two main advantages, first of all it doesn't need any "switched tapped coil" or "roller inductor" so it's easier to build, second the coarse tuning can be performed in RX mode, just by adjusting the two capacitor for "max noise" and then switching to TX and doing the final "fine adjustment", so it will help sparing batteries in portable mode !

If you'll decide to go on and try building one, I suggest following the approach shown here (see the schematic)

http://dl4cs.de/funktechnik/ant/fri-match/en.htm

the above adds a pair of swithes, one allowing to reverse the input/output to match hi-Z or lo-Z (instead of having to manually swap the connections) and a second one used to bypass the ATU (useful if you have low SWR on some bands so don't need the tuner), other than that, there's nothing particular, just a tapped coil (winding it on a toroid isn't a bad idea) and two variable capacitors

Getting back to your "random", did you consider carrying a fiberglass fishing rod with you ? A 33ft one will allow to lay out the antenna as inverted V or inverted L and once closed may be tied to a backpack and carried around quite easily

My reason for building this antenna was 2-fold. First, I wanted to prove out the QRP-level 9:1 unun design. I am satisfied with its performance. Second, I wanted to assemble a field antenna that would let me use the 160m band. By pairing the antenna with a wide-range tuner (in this case an LDG Z-11Pro II), I was able to tune and make a contact on 160m. The 160m objective is the reason I chose the slightly unwieldy radiator length.

I set the antenna up in an inverted V, using an arborist throw line over a tree branch to support the center of the radiator. Fortunately, my operational area has lots of trees. I do have a fiberglass mast for use when needed. It is rarely needed.

Thanks for all the help and suggestions. Again a more complete report will be coming soon.

Best 73 de Brian - K3ES

saw your post on QRPer

https://qrper.com/2023/02/k3es-puts-a-n ... field-day/

nice job I'd only add a choke to the coax, that will help a bit, since the coax braid, laying on the ground, isn't exactly a good radiator, better pushing all the RF you can through the antenna

as a note, if you're going to repeat the "experiment", did you consider installing something like those "ice fishing" cabins (or even a small container) ?

[edit]

just an example

https://www.home.crs/ideas/detail/diy-ice-fishing-hut

btw you won't need the hole on the floor but you may want to add a coax duct and probably an exhaust for a small stove

Thanks Andrew. Further details on the antenna build are coming soon, and I will link them here after they are posted. In a sneak peek, I had 3x17 ft ground radials deployed at the feedpoint.

Best 73 de Brian - K3ES

Brian - K3ES wrote: ↑Wed Feb 08, 2023 12:36 pm Thanks Andrew. Further details on the antenna build are coming soon, and I will link them here after they are posted. In a sneak peek, I had 3x17 ft ground radials deployed at the feedpoint.

Best 73 de Brian - K3ES

As for radials, the idea is that they must act as a low impedance return path for currents, so that with the help of the choke, there will be really little CMC on (the external surface of) the coax braid, now; reading some stuff here/there, the suggested length is below 1/4 wave (or around 1/8 wave) so I decided to quickly put together a piece of code to calculate 1/8 and 1/4 lambda for the ham bands and the results were the following (lengths in meters)
now, if you look at the above data, it's easy to notice that some lenghts repeat or, either, are pretty near, this means that a counterpoise system for an antenna covering all bands from 160 to 10 meters will require just three counterpoise wires (or radials if you prefer), that is
the 10m (33ft) wire (radial) will cover the 160, 80 and 40 meters bands, the others will cover the other bands, if you look at the data it's easy to see that the chosen lengths falls near either 1/8 or 1/4 wave for the various bands, and setting up such a counterpoise system won't require too much effort (willing to go all the way down the road you may add a fourth 20m wire for 160m, but I believe that the 10m one will suffice)

[edit]

forgot, if you're curious, you may explore your junkbox to find the bits and pieces to build this simple device

https://www.qsl.net/on7dy/Documentation ... Meter.html

it will allow you to check if there are CMCs on the coax and will also allow you to check how much current flows on the antenna radiator and on the counterpoises, using the three counterpoises I suggested, it will be easy, using that instrument, to check how the currents change on each of the three wires on the different bands

Grr... forgot, I posted the following as a comment on QRPer but it doesn't seem to show up, so I'll repost it here

A couple accessories you may consider when operating at low temperatures:

A "sack" shaped padded pouch with an elastic band around the opening and a small hole at the bottom; you insert the CW paddle inside the pouch slipping the cable outside through the hole, then you put your hand inside the pouch and operate the paddle w/o freezing your hand, optionally you may place one of those small hand warmers inside the pouch now, you'll probably tell me that you'll still need to use your hand to write contacts on the logbook... well, no, you don't the second accessory is a digital voice recorder, it may either be activated by voice or by hand (even if wearing a glove you should be able to activate/deactivate it with your left hand), you'll use the DVR to record the callsign, signal and whatever other info, then (possibly in a warm place) you'll listen to the voice recording and transfer the contact on paper or on a computer

Here's the link to my report on the 9:1 antenna build from over at QRPer.com:

https://qrper.com/2023/02/in-pursuit-of ... w-antenna/

I used my homebrew 9:1 EFRW with 144 ft wire last night in a POTA park. I was successful in making 3 contacts on 160m, and a 58 other contacts on 30m, 40m, and 80m.