Monday, September 9, 2019

ZS6BKW Antenna

One of the antenna projects I wanted to get finished off this summer was get the 40m OCFD up in a flat-top configuration.

Got thinking about how I'd lack an antenna that covers 75m NVIS, thought about an 80m OCFD again. At 135ft long, the current situation with tree shapes, sizes and locations leave that easier said than done, and not for a lack of trying with out resorting to chainsaws. Having said that, getting one of two 80ft Fir trees closest to the house removed is on the list, so I may be-able put up longer wire antennas in the future.

What else is there that's under 100ft, at a minimum covers 40 and 20m with out the need for an ATU, and will work on 75m?

After considering a few ideas, I took a closer look at the ZS6BKW antenna which is an optimized variant of the well known G5RV. This antenna checks the boxes on these good points:
  • Light weight - no traps, coax etc hanging.
  • Low SWR on 40 and 20m, can run ~1kW with the AL-80B.
  • Respectable gain on 20m if you have the luck or luxury of orientating the broadside to favor somewhere of interest.
  • Will work on 75m with the remote ATU I already have.
  • Also covers 17, 12, part 10 and 6m with low SWR. 17m opens occasionally during the current solar minimum, and there is the Puget Sound 10m net on Sunday nights I like to take part in.

Spent a couple weeks reading everything I could find, and playing with models - one is included with MMANA-GAL, others can be found online.

Decided it was worth trying out. Initially I was going to order the parts from The Wireman and build it my self, but found a source of these pre-made cheaper and better built for $60 (normally $90) plus shipping from Amateur Radio Supplies => ZS6BKW G5RV 80, so went that route. 

Current setup:
  • Antenna apex at 50ft, ends in inverted V configuration.
  • K9YC inspired choke balun - 7 turns of RG-8X through two mix 31 clamp on ferrites.
  • Remote ATU between the choke and coax to the shack. 

How does it work?
  • Except for 15m, low SWR on six bands from 40 to 12m, parts of 10 and 6m.
  • Remote ATU dials it in on 75, 60, and 15m, brings the total to nine bands.
  • 30m is the odd one out, ATU fails to find a match in my case.

Nine bands where only three need an ATU, with 93 feet of wire in the air is OK by me. 30m might be an issue for some, but you can't always win 'em all :-) 

Links to further information:

What's next?
  1. Get the ends up higher - the test inverted V setup I have has the ends coming down too steep and into the trees, as expected resonance / low SWR favors the low ends of the bands.
  2. Maybe after the trees are "trimmed" it's possible I could move the antenna to where the vertical is with the existing radial field. This would allow me to operate it as a T (top loaded vertical) by bridging the two legs of the ladder line together at the base, and feeding it against ground resulting in an ok 80m DX transmit antenna in place of the vertical, eliminating one antenna from my setup.

What about low band DX? Not much progress there, and with winter approaching need to sort something out. Putting up the K9AY receive loop and building an 80/40m matching system for the 43ft vertical, possibly returning it to an inverted L is the "quick fix" solution.

Monday, August 12, 2019

Bay of Islands ARC Repeater Site

Following on from the Auckland VHF Group Repeater Site post, some more pics from the archives. This time of the Bay of Islands ARC Repeater Site, photos I took from around 2007 when I had the task of reinstalling the equipment after it had been removed for re-alignment.

This site was established with the help of the Auckland VHF Group in the mid 90s to get the national link extended north via a sister site between here and Auckland called Brynderwyn. NZART map here - its approximate at best. Google Map that I created here, the locations are with-in tens of feet in most cases.

Since these photos were taken when I was last there, the 2m repeater has had some changes with the addition of CTCSS for IRLP node 6398 link, there are more recent pics on their homepage and some from when the tower was erected.


Pic of the tower and antennas. Like Klondyke, the 2m repeater antenna is constructed from angle iron, welded, and hot dip galvanized, but is a simple pair of 2 half waves in phase for around 10 dBi of gain, hard to see as they are end on in this pic. At the top is a 4 dipole stack for the the 70cm repeater. This configuration resulted in the 2m and 70cm repeater performance and coverage being closely matched in most cases. The link to Brynderwyn is a set of four 7 element Yagis with around 17 dBi? of gain, the Brynderwyn end has the same antenna configuration, this is needed because the 70 mile path is not ideal.


Inside the building.
Tait T300 gear.
146.750- also had a 50W PA in the back.
439.975- had the matching Tait 50W PA.

Thursday, July 4, 2019

New Desk

After using a folding table for the last few months, 'twas time for a something better.

I had considered The Great Ham Radio Desk Project, but after looking around IKEA I found something I liked better, more so since IKEA was having a sale at the time.

The spare room/office/shack is 120" wide across the back wall. What I ended up getting were three sets of draws and two table tops, the table tops are 59" x 29", the 29" width is nice, as it allows plenty of depth for radios, computers etc with space in-font.


The total was about $270 less tax and delivery fee, took an afternoon to put together.



Left to right, Ameritron AL-80B, Kenwood TH-D72A HT on top, Motorola MCS 2000 33cm/900MHz radio, Kenwood TM-731A, Icom IC-7300, 27" iMac (2014, 5K), under the iMac is a Windows 7 laptop which runs all the ham radio applications.

Plenty of room for more stuff! Plan to add some shelves to mount the Kenwood and Motorola mobile rigs, control boxes, and antenna switches etc.

Saturday, June 22, 2019

Summer 2019 Antenna Projects

40m OCFD: I built the classical 40m version with 33% offset noted previously. For a quick "throw it up and see" test I had it in an inverted V config, the SWR and resonance results are inline with expectations.

I fired up the Ameritron AL-80B a couple weekends ago with the OCFD. On 20m propagation was the worst I've come across, managed one contact into Missouri but had to use the Utah WebSDR to receive as signals were weak. On 40m I later made two contacts into Utah, conditions were tough with static crashes from thunderstorms.

Listening to my self via the WebSDR, the 10dB gain of the amp makes a big difference. Sounded clean and punchy with the Icom IC-7300 driving at 70W with the ALC connected. The T/R switching is via a solid state relay interface I built as suggested by KV5R.

Inverted L / 43 ft vertical: Restored to 43ft vertical for now, OCFD needed the only high support I have via a tree that the horizontal wire ran to. Interesting note with the vertical, when it was last simply a vertical I had the radials laying on the ground, the SWR/match was a bit "gimmicky" in on some bands where for example on 20m, the remote ATU failed to find a match. With them buried 2" to 6" deep things are much more normalized.

What's Next?

Get 40m OCFD up in flat top config, and dialed in. At near 50ft it will cover 40m for NVIS, 20m with a good pattern and low take off angle, plus 10m and 6m. I might move the feed point to 40% and gain 15m coverage.

I got some mast sections from The Mast Company, plan to install with an apex eave mount. Put the Comet GP-15 on the top.

40m DX Option 1. The mast may provide a support for one end of a 40m Half Square. Will favor SW / NE which covers the Pacific, and Europe / Africa in the other direction. More info regarding half-squares:


Far field plot for the half square I modeled.

40m DX Option 2. Two element vertical Yagi using a single 60ft support, I got the idea from this page - http://dl2kq.de/mmana/4-3-35.htm. I have tweaked the geometry to bring it up to 7.15MHz.

I found additional information by N6LF - Getting the Most from Half-Wave Sloper Arrays.

There are perhaps 3 ways to feed this antenna:
  • N6LF settled on resonating both elements as reflectors, and end feeding the selected driven element - the parallel tuned circuit used to match takes care of resonating the driven element.
  • Borrowing from K9YC's End-Feeding a Center-Fed Vertical Dipole, the bottom half of each element would be made from coax with good chokes at the bottom which K9YC also discusses. Leaving one side open circuit resonates the element as a reflector - the length of coax is critical, I calculated (info below) there would be enough length to form the bottom half of the dipole, the choke and connections to switching relay. Some form of strain relief may be needed for the coax half of the dipoles to stop it from stretching.
  • Using the same principal as above but center feeding with coax (just like a conventional coax fed dipole) to a mast mounted switch that short circuits the inactive element creating a closed stub which resonates it as a reflector, the length needed is much shorter hence the mast mounted switch. A balun would be needed at the feed point which adds weight.

The latter two open/closed circuit stubs will present an inductive reactance lowering the resonant frequency of the passive element turning it into a reflector. How to figure it out? W8WWV wrote a nice article on Coaxial Cable Stub Q, which lead me to an application called Transmission Line Details. Among other things this can be used to calculate the length of open or closed stubs, it also tells you how much R the stub will add at the feed-point of the passive element - this information can be plugged into MMANA to know the effect of the added R.

Another handy calculator I found is Inductive Reactance Calculator, which converts an inductance at a given frequency into an inductive reactance value. Useful in conjunction with MMANA-GAL and TLD.

It's important to note that properties of coax varies between manufactures and manufacturing runs. The length is critical, one would need to measure the velocity factor of the coax on hand using an antenna analyzer, not rely on published information. I suspect this is the reason why some struggle to get phased arrays to work properly, relying on the publish spec is not close enough to reality.

Personally I think N6LF's end fed might be the better choice as it eliminates the weight of coax, baluns, mast mounted switches etc. If using high power, capacitors need to handle several kV. The only critical part is resonating both elements as reflectors.

From the model, I like this antenna for a couple reasons, it has a nice broad beam width, and 2 dBi of gain at 10 degrees elevation, the F/B is ok at 13 dB. It can also be built using a single support.

MMANA-GAL file for 40m 2 element vertical Yagi.


View of 2 element vertical, these are half wave dipoles with the ends angled towards each other. 


Far field plot for the 2 element vertical.


3D far field plot from GAL-ANA.

What I like about this is it can be built with a single 60ft support. Has good gain at low angles, 2 dBi at 10 degrees, -3 dB beam width is slightly more than 120 degrees.

40m DX Option 3. Update 08-Aug-19, attempted to build this with my 43 ft vertical, however the vertical is too flimsy to support the tension needed on the passive elements to get them into the right shape. I'd need to rebuild the vertical with longer sections (less taper) and guy it. I did enjoy the time spent researching this antenna, modeling it, and learned a few things along the way. I'm disappointed that I cannot make a good attempt at building this antenna. Maybe another day!

Another variation of vertical Yagi from http://dl2kq.de/mmana/4-3-35.htm is the 3 element GP80 scaled for 40m.

Very close resemblance to the Spitfire antenna developed by K1VR and W1FV - http://www.yccc.org/Articles/Spitfire/spitfire.htm. It also appears in ON4UN’s Low Band DX book, and N6LF's Single Support Gain Antennas for 80 and 160 Meters.

What I like about this is the 43ft vertical I already have with 32 buried radials is the right length for the 40m version driven element. Makes good use of what I already have in place. All I need to do is add the passive elements, and simple matching network with series capacitance.

This antenna design is kinda neat on a few points:
  • The driven element is around 1/3 λ this raises feed-point impedance to a convenient 50 Ω, and can be adjusted as needed to be 50 Ω, series capacitance takes care of bringing it to resonance
  • The extra length also supports the passive elements and their shape, can double as guy lines.
  • The passive elements are 1/2 λ and folded (and thus independent of the ground plane), shortening one via switch/relay in the low horizontal section turns it into a director, direction could easily be made electronically reversible.





View and far field plots of the 3 element GP.

MMANA-GAL file for 3 element GP 40.

Since I have the driven element already in place, modeling it by it-self showed feed point being 70Ω, jX 108 at 7.15 MHz. I went and measured it with the analyzer and to my surprise it's very close:



Analyzer shows 72Ω, jX 104, along with the series capacitance needed for resonance - 213pF. Adding series capacitance of 200pF to the model brought it to resonance. The test and result is encouraging.

What to do for 80m due to space and tree locations, I've considered many different possibilities. The best solution I can find is a quad loop squashed into a rectangle. Feeding from one of the sides results in bi-directional low angle vertical polarization. An 160m version of this antenna gets a mention in Low-Band DXing 5th Ed - Chapter 10, pages 4 and 5.

At the other end of the spectrum, the 7 element Yagi I have in the attic for 900MHz is sub-optimal, need to relocate it or try something different.

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.

Having recently acquired a used Ameritron AL-80B from VA7ST, I also now need an antenna with a good match on 40 and 20m that will take some power - up-to 1kW PEP. My LDG remote ATU is rated to 600W PEP, at high(er) power levels I'd prefer to have properly matched antennas available.

I also bought An Introduction to Antenna Modeling with my $10 birthday coupon from the ARRL, this has good info for getting started with antenna modeling. With some ideas in mind from the book and what ON4AA described with their 80m center loaded OCFD design, I set a design goal of a good match (under 1.8:1) on 40 and 20m..

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 at 14.2MHz = 10.45m.
  3. Double the length to 20.9m.
  4. Tried various offsets between 17% and 40%.
  5. Found 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 above slightly for better formatting. Now, time for some pretty charts:

40m
20m
10m
6m

MMANA-GAL file for the 40m OCFD with 33% offset.

Next step, build it and see!

Side Notes:
  • The averaged R across the four bands is 157 ohms, using a 3:1 balun would get [in theory since this is a model after all!] a very good match.
  • On 40m, resonance is just below the band. Borrowing from ON4AA's center loaded design using a capacitor in the center to resonate the antenna on the lowest band, 425pF did the trick in the model.
  • MMANA-GAL file for 40m OCFD with 33% offset and center loading.
  • MMANA-GAL file for 40m OCFD with 40% offset, this gains 15m band coverage at the expense of higher SWR on 40 and 6m.
  • More experimentation may improve things further..

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.