Saturday, May 25, 2019

Low Band DX - 5 - 40m OCFD

After previously re-considering verticals or dipoles, I decided to build an OCFD using ON4AA's design and ordered a Model 4116 balun from Balun Designs LLC.

To start with, I have decided to keep things simple by comparing verticals and dipoles on 40m. I could put up a simple 40m dipole, but had this balun already.

Looking at 40m OCFD designs, none seemed to be that great. SWR over 2:1 on 40m, or different offsets to get 15m resulting in worse matches on other bands.

I have also recently acquired a used Ameritron AL-80B from VA7ST, and at the very least want an antenna with a good match on 40 and 20m that would take some power.

Having gotten a copy of An Introduction to Antenna Modeling with my $10 birthday coupon from the ARRL which has shown me how to better use MMANA-GAL, maybe I could find something that met the design goal of good match (under 1.8:1) on 40 and 20m with the help of modeling..

The process I followed:
  1. MMANA-GAL, using real ground, and a height of 12m / 39 ft (my situation).
  2. I started by finding the resonant length for a 20m dipole (14.2 MHz) = 10.45m.
  3. Double the length (20.9m).
  4. Tried various offsets between 17% and 40%.
  5. 33% is the best compromise between 40 and 20m.
  6. An added bonus is low SWR on 28.5MHz and 50.1MHz.
Results:

07.15 MHz, R 144, jX 74.6, SWR 1.72, Ga dBi 5.58, Elev 51.1
14.20 MHz, R 122, jX 2.39, SWR 1.64, Ga dBi 8.12, Elev 24.5
28.50 MHz, R 162, jX 14.8, SWR 1.25, Ga dBi 9.44, Elev 12.3
50.10 MHz, R 202, jX 32.1, SWR 1.17, Ga dBi 11.2, Elev 7.00

I have rounded the numbers slightly for better formatting.

40m
20m
10m
6m


MMANA-GAL file for 40m OCFD 33%.

Next step, build the 20.9m OCFD with 33% offset and see!

Side Notes:

  • On 40m, resonance is just below the band. Wondering if I could achieve near perfection borrowing from ON4AA's design using a capacitor in the center to resonate the antenna on the lowest band, 350pF did the trick in the model. It also raised the resonant frequency a little on 20 thru 6m.
  • The average R across the four bands is 157 ohms, using a 3:1 balun would get - in theory since this is modeled - a very good match.

Saturday, April 27, 2019

WSPR - 40m, ZL5A Scott Base Antarctica

WSPR pulls out another surprise.

Adam Campbell / ZL5A from Scott Base Antartica:




Wednesday, April 17, 2019

Low Band DX - 4 - Vertical or Dipole?

Vertical or horizontal dipole? Excellent question, since I began my low band DX quest, I had assumed horizontal dipoles are NVIS or short hop only antennas, due to the effort required to elevate them 1/4 wave length or more above the ground.

A few days ago I started looking at phased verticals to make some gain, and had settled on a plan to build two 80/40m trapped verticals based on W8WWV - Hex Array - 80/40 Meter Vertical, spaced 33 feet apart - 1/8 wave on 80, 1/4 wave on 40, and setup phasing for 135 degree phase shift on 80, and 90 degree phase shift on 40 with the option to reverse the phase to switch directions.

While scouring the internet for information, I ran across Verticals: got two? by N4JTE. Ok I thought, Bob had success with this, then I took at look at Bob's QRZ page and discovered that Bob had discovered something even better, the same thing but horizontal!

A while ago I noticed something interesting when comparing radiation patterns for verticals and horizontal dipoles, on first glance one concludes vertical has all the power down low. But when I compared the actual gain figures at 10, 20 and 30 degrees elevation, the dipole, even when under 1/4 wave length high has more gain. I dismissed it at the time thinking, I must be overlooking something.

¯\_(ツ)_/¯

I started looking into this again and found:



Above from http://www.chem.hawaii.edu/uham/portant.html comparing antennas on 15m:
  • A very highly optimized vertical. It uses 16 elevated radials 28 inches off the ground.
  • Horizontal fan dipole (0.3 dB better than regular dipole) at 15 feet above ground, 15ft on 15m band is between 1/4 and 1/2 wave length.
What's eating the vertical's energy? That null up top should be pushing more out at a low angle.. Ground losses against the return currents to the base of the vertical? Only when verticals are over salt water does the low angle lobe "push" right out.

Now what? I'm not tearing down the inverted L just yet, but I'll be building an OCFD based on the ON4AA design with the center loading network for 80m. The balun I'll be using is Model 4116 - 4:1 Hybrid Balun 1.5 - 54MHz 3kW from Balun Designs LLC. I have built OCFDs before with Balun Designs OCF optimized baluns, and had pleasing results.

The fun part is I'll be-able to compare the two antennas for a while.

After taking a ZL-Special model in MMANA and dropping it down 1/8 to 1/4 wave length above ground, it suggests the gain will be several dB better than a vertical at low angles.

Ultimately I'd like build something similar to Bob's double whammy with 40 and 80m capability. From my location in the Pacific North West, orientating it for South West / North East it'll favor the Pacific, ZL/VK, Europe and Africa.

Another advantage of horizontal polarization = less local noise pickup, will the RDF exceed small receiving loops? Since I have a K9AY loop control box, I'll also be-able to compare these too.

Saturday, April 13, 2019

The 900MHz Club

After getting to know a new group in my area on the local N7OEP 70cm/6m linked system, there is occasional chatter about "the 900MHz club". Otherwise known as the 33 centimeter band.

ZL has an allocation from 921 to 928MHz, commercial two way radio gear is non existent that I know of except for the digital Tetra system around 860MHz. If there was analogue FM stuff around to modify, the commercial 25MHz repeater splits are too wide for the band. As far as I know, there is no activity on this band in ZL bar maybe a mad scientist or two frankensteining something together in their basement shacks. Those into things above 70cm start at 1296MHz and up into the microwave bands from there.

The US has 902 to 928MHz allocated, and here in Western Washington we have several 900MHz repeaters and some activity. Getting on 900MHz requires sourcing, reprogramming and sometimes making physical modifications to commercial 900MHz two-way radios.

I ordered two Motorola MCS 2000s from Used Radios, they shipped quickly, were well packaged and the two I got appear to be in good condition. The pair I got are the version 3 model that also make 35 Watts, 35 Watts at near microwave frequencies is rather respectable. Here in the Puget Sound area there are a couple of people happy to program the Motorola rigs. As for antenna's plenty of cellular stuff around that covers this band, cheap Yagi's can be found on e-bay and Amazon, or in the spirit of ham radio, make something! I ordered a couple 7 element 900MHz Yagis to try out at $16 each.

 

See Getting Started with 900Mhz (33cm) Ham Band by WA6AER for more info about whats what. The largest hurdle with this stuff is getting the radios programmed, either your self or by someone else.

Whats it like? Well like FM anywhere else except using a commercial Motorola radio gives things an industrial feel, no S meter, basic controls - on/off, volume, memory up/down, and a bunch of buttons that do nothing.



Time for something bigger and better than a folding table.

Monday, April 1, 2019

Low Band DX - 3 - Feed Line Routing and Grounding

As with any antenna project, appropriate cable routing and grounding is fairly important.

After reading some very educational and informative articles written by W8JI:


I realized I had some work to do, originally I had run my HF antenna feed line up the wall on the other side of the house, through the attic and down an internal wall into the shack. I had installed a ground rod at the far end of the feed line where the remote ATU was located when I had the Carolina Windom up - with that setup I had created my self a ground loop, a lightning protection problem and an electrical safety issue - House Ground Layouts has several diagrams of how to get this wrong, and how to get it right.

With that I routed my feed line around near the electrical service entrance burying it couple inches underground. At the service entrance I repurposed an old PrimeStar box I found in a pile of junk when we bought the house, along with an old PrimeStar Ku band dish thats now hanging up in the garage awaiting a project...

 

The copper strip I found at Lowe's in the plumbing section, this had the right sized holes for bulkhead N-type connectors, its actually lightly copper plated steel, I gave it a light coating of Jet-Lube SS-30 Pure Copper Anti-Seize to prevent rust and so the connectors would make a good electrical connection. The right angle N-type connectors are a little hard to find at a good price, HRO had them for about $6 each. The green wire runs about 3 feet over to the electrical service entrance grounding rods - I plan to upgrade this to something better such as a copper strip or heavy gauge wire later.

 

Feed lines run up the wall into a junction box I got from Lowe's, it allows for enough of a bend radius with LMR 400 to then go through the wall. The shack is on the upper level. A little work left to tidy things up nice.


On the inside I used a standard 4 port Keystone wall plate designed to take the Keystone snap-in connectors such as the RJ-45 bottom right - I plan to use that with CAT5 cable for K9AY loop control cable. The N-type bulk head connectors were just right with the o-ring and washer removed to allow the connector on the other side to screw up tight. Looks nice eh?!



Bonus pic, while I was walking around with the camera phone, a bumble bee had flown through the garage to the window and buzzed up the spider web. Bumble bee made a lucky escape and flew away back out the garage door as I mounted a rescue effort. You can see a large spider made an appearance in anticipation of some lunch, in-fact at least two of these guys live here, there was another one to the right just out of view!

Friday, March 22, 2019

Low Band DX - 2 - Vertical to Inverted L

Last weekend I decided to upgrade the vertical to an inverted L, specifically a 1/4 wave on 80m.

Measuring the 43 ft vertical purchased from ZeroFive, it has 14 x 32" sections and the base section is 36" for a total of 484" or 40.3 ft - where is the other 3 feet? No idea! Running some rough calculations I added a 22 ft horizontal wire to the top, and after a fight with the maple tree snagging the wire a few times I got it up.

I was a little surprised at the changes, SWR:
  • 80m, 3.5 MHz is near 1:1 and rises towards the top end of the band.
  • 60m, SWR high, remote ATU fails to find a match.
  • 40m through 10m SWR high, remote ATU matches it in no problem.
Since I have no real idea as to what load the antenna is presenting to the ATU, its hard to know much more, to solve that I have a RigExpert AA-55 antenna analyzer ordered.

With that, I decided to bang my head on antenna modeling to help get a better understanding on what to expect from this inverted L I have created. Being brand new to modeling software, I found 4NEC2 "too hard". I took at a look at MMANA-GAL, which includes an inverted L file. After a quick edit of the geometry to match what I have, the results were interesting. Granted I might have missed an important detail with this, it could easily be totally wrong.

Starting with 80m and working our way up, blue is horizontal, red is vertical polarization:

80m, what I expected, the small horizontal component is from the horizontal wire at the top. An anecdotal observation is the small horizontal component has made receiving local signals via NVIS stronger.


40m, calculating the length on 40 turns out to be a 3/8 wave, so that works.

For reference, here is the contents of the MMANA-GAL file:

Inverted L: v40 ft + h22 ft
*
3.75
***Wires***
2
0.0, 0.0, 0.0, 0.0, 0.0, 12.28, 0.015, -1
0.0, 0.0, 12.28, 6.7, 0.0, 12.28, 0.001, -1
***Source***
1, 0
w1b, 0.0, 1.0
***Load***
2, 0
w1b, 0, 0.0, 400.0, 0.0
w1b, 1, 10.0, 0.0
***Segmentation***
400, 40, 2.0, 2
***G/H/M/R/AzEl/X***
2, 0.0, 4, 50.0, 120, 60, 0.0
###Comment###
Feeding direct coax 50 Ohm.
Bandwidth (SWR<2) 210 kHz.
Load 2 ia a ground loss.

Tuesday, March 19, 2019

WSPR - 40m, 1 Watt, 16441 km

Distance wise, this is the 2nd best spot I've reported yet at 16441 km / 10216 mi, from ZS1LCD in Cape Town South Africa who was running 1 watt on 40m [7MHz]. ZS1OA remains king at 16511 km / 10259 mi on 30m [10MHz], and ZS3D is a regular in 3rd place at 16233 km / 10087 mi on 40m.


After running WSPR for about three months, I took a break from it about a week ago having built up a good feel for what I can expect with the current antenna configuration. Last night I had it running again after adding a 20 ft horizontal wire to the top of the 43 ft vertical turning it into a 1/4 wave inverted L on 80m.

With the exception of 60m, Remote ATU has no problem finding 1:1 match 80 thru 10m, 40m was the surprise since the antenna is close to a 1/2 wave. Hard to know with out an antenna analyzer to see what is really going on. I'll soon have a better idea, ordered a RigExpert AA-55 analyzer, and another 40 x 32 ft radials for the vertical / inverted L to bring the total to 60.