Friday, August 15, 2014

Counting - Radio #2 - Yaesu FT-817ND - 20m Delta Loop - Cheap, Lightweight, Portable and with Gain

I have been asked before if I had to take the minimum amount of radio equipment to a remote location what would I choose.
Chatting about this with other radio amateurs one can easily conclude that there aren't identical answers. Everyone has it's own preference, so what I am about to write is my *own* preference.
Here is goes:

  • The Yaesu FT-817ND. Very nice little radio. All bands, all modes, in a very tiny and competent package.

  • For working several bands you cannot go wrong with the LDG Z-817 tuner. It's a nice match with the FT-817 and with the interface cable it all comes together extremely well (in a all automatic fashion).

  • If you hate automatic tuners and you want a cheap but efficient option, then my choice goes to the beautiful QRP Projects ZM-4.

  • For the power source, I usually go with a 12V/7Ah SLA battery, although on a really remote location I would also take the PowerFilm F15-600 10w Folding Solar Panel Charger, good for 10 watts of power, at 600mA.

Now, it's when it comes to the antenna that we all make different choices.
I have spent many hours looking for a cheap, easy to build, easy to carry, easy to erect, with gain antenna to conclude that the best compromise possible is the 20m Delta Loop. This antenna, when properly built, will present itself as a 50 ohm load at the resonant frequency and it will offer you 2.78 dBi gain, which is 0.65 dBd gain, in freespace. It will also work (with a tuner) on 40, 15 and 10m.
Here's how my loop is setup and what what you will need to build one yourself.

Resonant loop antennas have a feedpoint impedance of about 100 ohm. There are many ways to feed a loop antenna. For me, the most practical way, is to use a 4:1 current balun. On my loop I am using a commercial balun, a 4:1 Guanella current rated at 150 watts pep, 50 -200 ohms. The one I am using is from G-Whip Antenna products and at about 25£ it will not break the bank. It's also a quality product with stainless steel hardware, designed to last on permanent installations.

Now, for feeding the delta loop antenna you have several choices:

I went for option D, Apex Up, Corner Fed. This setup allows Vertical Polarization. This will give you a low radiation angle, which is the best choice for DX. It also has the advantage of requiring only one central support as I will illustrate latter in this post.

Calculating the loop total size is actually quite easy. To make things even easier you can use this online loop calculator that will do all the math for you.

For 20m here's what I got:

To be more precise, for 20m, mid band (14.175MHz):

Total Wire Length: 21.61m
Delta Loop, one Side: 7.203m
Height (Base to Apex): 6.238m

Although it's true that feeding the loop at the bottom corner will give you predominantly vertical polarization, feeding it a quarter wave down from the top middle is theoretically better.

Since each size is 1/3 of the total length, finding the correct feeding spot is quite easy. Just divide the full length by 4 to get the necessary 1/4 measure:

1/4 of the total length is 21.61m / 4 = 5,403m

So, your feed point location would be 5,403m measured from the apex, or, if you prefer:

7.203m - 5,403m = 1,80m

That would be 1,80m from the base (corner point) to the feed point.

Finally you need a practical way to set the loop up. A cheap fiberglass pole is the right way to go. My choice is the Essential Pole 700 from Caperlan. It's 7m when extended although the last 1m section may be a bit to fragile for antenna usage,

And here's how it all goes together (10 minutes is about the time one takes to erect this antenna).
The complete antenna:

The 4:1 balun:

The mast, when extended:

And, of course, when properly packed this is an antenna that fits nicely into a small plastic bag.

So, there you have it. Cheap, small, lightweight, easy to carry and with some gain. My favorite simple antenna for portable operation. What's yours?

PS: For some QRP Delta Loop action make sure you watch Peter, VK3YE video on YouTube.

No comments:

Post a Comment