YO3BN - Portable Operation on 80m Band

Posted at - 21st August 2017
Updated at - 4th September 2017

Portable Operation on 80m Band

During 15th - 20th August 2017, I was portable from Lehliu/Romania - KN34KL, a place where I made several experiments and many QSOs. Equipment used: my SSB homemade transceiver for 80m, my portable antenna for 80m, 50W QRO when powerline electricity was available, EMF meter, SWR bridge, a coil, a variable capacitor, a common mode current choke, LiIon pack, etc.

The antenna system can be assembled or disassembled by a single person within 20 minutes. It weighs approx. 3kg and consists mainly from the following parts:
the mast:

6m fiberglass fishpole

the mast support:

4 tent stakes
2 guy ropes

a halfwave wire dipole:

2x20m PVC 1.5mm² copper stranded wire
2 homemade egg insulators
2 guy ropes
2 tent stakes

the feeding system:

14m of 2x0.75mm² speaker cable
common mode current choke
an L antenna tuner

Experiments and Notes

First try with this portable antenna I've got almost 59 with 50W on the air, so I felt something is a bit wrong because several months ago I've got 59 using only 4W RF output and a similar antenna. From my base QTH I've got about 59+20 with 50W on a 400km radius. After some sort of inspection I found the cause, the dipole ends were too close to the ground. I lifted them up about 2.5m above the ground making a huge difference and since then I've got 59+10 +20 reports.

With this portable setup and 4W RF out I was able to communicate over 270km radius on 80m band in a late morning.

Measuring electric field around the dipole wire with my EMF meter, readings confirmed maximum voltage at the ends and minimum at its center.

Placing my finger tip at the dipole end I felt a very strong burn sensation accompained by small discharging sparks even if the dipole was feed only with 4W though!!

Placing a halfwave wire below and parallel with the dipole does not seem to improve NVIS radiation, but it will copy the voltage from the dipole at a much lower magnitude. So, I think that the two wires are too close to each other (about 5-6m), I need to calculate the distance between the dipole and its reflector in order to find the optimum distance between them on which the radiated waves add them constructively, or see Yagi patterns.

I've found that my 2x0.75mm²speaker cable, which feeds the dipole, radiates some electric field. I need to make a test between twin lead and coaxial cable using transceiver, cable, dummyload and my EMF meter to found which one radiates more or less.

Other Photos

Update - 4th September 2017

NEC Analysis

I was curious about how better my antenna performed in the field, therefore I have learned to use NEC simulator. Many thanks to Morel Grunberg 4X1AD for pointing it out to me. For analysis I've used 4NEC2 for Windows, emulated with WINE on GNU/Linux.

First, with the inverted Vee with ends very close to ground (about 0.2m) within a couple of QSOs I felt that something is wrong with this antenna, everybody gave me 58-59 signal reports with 50W output power, which should not be quite right because I know even with 4W and a higher inverted Vee antenna it will perform better than this. Shortly after, I wondered what should I do to improve my antenna, so a good point was to raise wire ends even more above the ground.
Let's take a look at NEC comparison, inverted Vee 2x20m center at 6m height, ends at 0.2m - red, ends at 2.5m - blue:

It can be seen an improvement about 2.5dB between them, not much, but in the air I think it performed better because my signal reports raised to 59+10dB.

Also, the complex impedance were simulated as follows by NEC:

348 + j2364 -> for close ends to the ground inverted Vee
214 + j1992 -> for high above the ground inverted Vee

Another test was to find out where the antenna resonates for 50 ohms, so the ATU was removed from circuit. Only SWR bridge, common current choke and transceiver were left at the antenna input. Well, I was surprised to see that it resonates somewere at 1.8-2MHz, much lower than expected!! Plotting SWR with NEC, proved this case too:

Unfortunately, I did not remember exactly where the antenna was resonating but I remember somere below 2MHz, I think the feeder and common current choke lowered the resonance further more.

The NEC models can be found here.

Update conclusions

The lower the antenna, the bigger the attenuation.
The lower the antenna, the lower it will resonate.

73 de YO3BN