Discussion in 'Amateur Radio News' started by K0LWC, Dec 20, 2020.
Check out this video that has an interview with G0GSF former ZS6BKW.
The folded version provides two vertical segments that provide endfire gain at low angles on 20 meters - a much better pattern than if the antenna is used in inverted vee form. The tuning doesn't change much between versions, but the most critical variable is the electrical length of the feeder line. Note also that using insulated wire may change the lengths of elements by a two or three percent - that's nearly half a meter or well over a foot! If in doubt, refer to G0GSF's article and other sources on the electrical length of insulated wire.
KG5LTL & AF7ON , thank you for the quick responses.
I have viewed the interview you attached (great interview) and read every reference I can find to this antenna on the Internet.
I have been effectively in COVID lockdown / working from home for most of 2020 so I've done a lot of research.
Unfortunately there is very little practical experience out there, most people are republishing existing material with little real world , "this is how I did it" experience available.
Interestingly there are many variations on this same antenna out there (the published lengths vary) and of course it varies depending on the type of material used and the height of the installation.
I've built the antenna as per Brian Austin's published articles, but are now looking for practical experience on how to "Tune" the antenna.
Critically, the feeder needs to be 0.62 wavelengths long (on 20M) and therefore the physical length of the open wire feeder varies depending on the quality and type of the feeder used. Mine is home made ladder line at approx. 450 ohm..
All variations, including the original G5RV, focus on measuring the actual VF of the Feeder and then cutting the feeder to length, before assembling the feeder to the central dipole support.
The issue is now the feeder is a fixed length and cannot be adjusted to align the actual antenna to the build and physical conditions of the site.
(Picture from Google Search)
I've borrowed this idea from a company (Trueladderline) and I've used a single length of plastic covered washing line (60m) and PVC tube with hole drilled in it, to allow both L1 and L2 to be adjusted as required.
From what I can gather "Tuning" is as follows:
Adjusting L1 (the dipole) to get it to dip VSWR as low as possible on the section of the band I'm after.
The excess dipole length (past the insulator) is laid flat against the dipole and wrapped with some electrical tape temporarily , so I can adjust long or short as required.
Then adjust L2 (the 450 ohm feeder) to get as close to 50 ohm feed match as seen at the 1:1 balun at then end of the feeder. (I'm using an eBay device called a Mini60)
Shortening the Ladder Line is as easy as marking a length with a permanent marker (Say 100mm) and then pushing the cable back through the PVC tube (with holes drilled in it).
The length the ladder line is shortened (100mm) is then marked on the dipole at each insulator and it is then pushed through the end insulator (PVC pipe with holes drilled in it)
Repeat process as required noting that you may need to extend the dipole / ladder line length as necessary
So, my questions are:
Does what I've written above make sense?
Is 20M (14.1Mhz) the right band to commence tuning against or should it be 40M (7.1Mhz) ?
Is there another frequency (not in a ham band) that is considered the "Fundamental" frequency for this antenna design that would be a better choice as the baseline frequency?
Is there a simpler way to "tune" the antenna, taking into account that the design does not have a cut at the feed line so the normal VF measurement practice does not work.
Sydney - Australia
I have found the same antenna(your choice) in different locations work differently. Depending on whats in the ground. What is around you and your antenna(buildings,trees or mountain range ) . Electrical interference. So on So forth. Just one antenna for every one isn't the answer.
We are A community of experimenters , so go for it . Try different things and see what works for you where you live.
You and I obviously have very different ideas about what "limited space" means. Lol
Can't wait to operate portable, really looking forward to putting up a ZS6BKW.
At HF frequencies these measurements are not at all super critical and lots depends on other factors like the earth under which the antenna sits and the surrounding objects and height abound ground and feedline length to your shack etc.
Just follow the basics. 46 feet of wire each side of the center insulator. 39.5 feet of ladder line. An RF Choke and coax to your shack. Don’t over analyze it ! You can easily trim the ladder line to resonance where ever you like to operate in the band.
These things are simple to make and modify. You don’t gain any measurable advantage by being perfectly resonant either.
Over the weekend I was able to test the antenna in a very quite radio location about 200km (125M) West of Sydney.(Grid = QF46IH)
I was able to get it up to almost 20 meters (65 feet) [about 1/2 the height of the gum tree] with the ends at approx 15 meters (50feet).
My measurements are 28.4 meters (97.13feet) with the ladderline of 12.2 meter (40 feet) and I have a 1:1 Balun.
The results (best dip) are:
Frequency 2.91Mhz 3.060Mhz 3.240Mhz
VSWR 1.98 1.37 2.00
6.3Mhz 6.610Mhz 7.070Mhz
2.02 1.50 1.99
12.680Mhz 12.910Mhz 13.300Mhz
1.99 1.8 2.00
26.920Mhz 28.00Mhz 28.820Mhz
2.00 1.71 2.01
49.060Mhz 50.420Mhz 51.400Mhz
2.00 1.25 1.30
Based on the numbers above I believe the dipole section is long for 80/40/20.
Still looking to get an answer as to what frequency to "Tune" the antenna as the fundamental Freq ?
And how to measure the VF of the Ladderline?
Please note that I'm using PVC coated Steel Wire Washing line. (60 meter roll)
If anyone can provide a VF for Steel Wire rather than copper , would be appreciated.
Thanks for responding Joe,
Can you assist me with some of the Basics ?
Such as do you adjust L1 (Dipole Section) to get it to the correct Frequency (Lowest SWR on the section of the band you want),
and then adjust the ladder line for a deeper dip ( resonance) , or do you do the reverse.
Is tuning performed at the Primary frequency, and then ladder line length to bring the rest of the bands into resonance ?
What is the process by which you tune one of the these antennas ?
The dipole wires and the feedline are combined into an antenna "system". Let us say that the antenna "system" can be tuned a number of ways. There is not a "right or wrong" way to tune them. You can either tune the antenna by adjusting the lengths of the L1, or you can adjust the length of the ladder line L2. You could even do both. The end result is quite similar.
Practically speaking, it is far easier to trim the ladder line a bit - which is usually done safely at ground level and without the need to erect and dismantle, erect and dismantle the antenna multiple times - to adjust to the portion of the band that you want to cover. Now the reality is that the ZS6BKW by design typically will cover the entire ham bands that it was designed for (ergo 40/20/17/12 and the higher end of 10M) with reasonable VSWR with no need for lots of pruning or tuning. At most you would add perhaps 30 cm of ladder line or subtract 30 cm of ladder line to dial in the VSWR to the specific frequency that you operate. That's the magic of the 46 feet length and the 39.5 feet ladder line. It so happens to be 50 ohms or close to it on ALL those bands and so there is minimal pruning needed.
The tuning steps may depend if it is a flat top T or an inverted V. If inverted v, changing the angle of the opening can alter the tuning. So can height above earth that the ends are oriented. If the antenna resonates low in the band and you need to go higher (say you are SSB only and the antenna resonates best in the CW sub band) then trim a few cm of ladder line and try it. I usually shoot for best match on the band / frequency that I operate most. But I don't fixate on that - with a ZS6BKW anything below 1.5:1 is good enough and even 2:1 is fine and canuse an ATU in the shack. Adjusting the lowest band, ie 40M will then impact all the higher bands. A small adjustment on 40M will have a 2x impact on 20M and a 4x impact on 10M. So do your trimming in small increments and ladder line is much easier to add to than each of the L1 elements if you do happen to clip too much off.
Ignore the performance on 75/80M band -- it was never designed to cover that band. Let us start with 40M. Your antenna is too long. You should be at 46 feet per side (total length 92 feet across not 97.13...you're too long) and 39.5 feet of ladder line (not 40 ft, again too long). Both "too long" situations are additive and cause the resonant point to be below the ham bands.
So as expected this antenna resonates far below 7 Mhz at around 6.5 mhz.
Dial in the 7 Mhz section first. That will impact the upper bands. Aim for minimum vswr at 7.0 Mhz or 7.1 mhz depending what mode you run in Oz (CW or SSB).
I suggest you cut the L1 wires to 46 feet per side. Then erect the antenna. Test the VSWR. It should now resonate closer to 7 mhz (higher in frequency than before). Do Not cut the ladder line til you test it out. If it is still below 7 mhz, now cut the ladder line to 39.5 ft. Then test again. IF it still is too low in the band, cut 2 or 3 cms at a time on the ladder line and re-test. You'll dial in 7 mhz first. Then 14/18/28 should fall into place.