Broadband antenna with gain on high bands
Dear,
Recently I came accross the interesting information about Davis Station on Antarctica (VK0JLX)
https://www.qrz.com/db/VK0JLX
They are using commercial antennas made by RFS
The receiving antennas are ST-330s made by the company. At VK0JLS they call it sloping triangles, but essentially the antennas are terminated V-beams (2 longwires) sloping down from a center mast.
I found that interesting enough to do some simulation work with NEC2.
My findings:
- It seems to me, that the low horizontal wire which connects the ends of the sloping wires from the mast (with the ST-330 they are 100m long) are functionally NOT necessary. But if it is difficult to make good and stable earth connections to terminate the sloping wires where they touch the ground level, the horizontal wire is a proper way to terminate, without any earth connection.
- The antenna has a very good low angle pattern, not many side lobes and seem to be rather similar as a large log periodic beam. It sounds very quit and is therefore excellent for reception. Although you can use it for transmit as well, the efficiency is not the best (due to large bandwidth and losses in the termination resistors). However for reception it is hardly necessary to use a pre-amp. Nor is it even possible to use a pre-amp here in Europe, because broadcast stations from 15/17Mhz band are extremely loud - you can check it out on my online kiwi (see URL below)
- >15Mhz vertical radiation angles are around 10deg or even lower!
- The termination resistor with the RFS original design of their product ST330 is about 800Ohm in the middle of the horizontal wire which connects the ends of the sloping wire. This is also the feedpoint resistance. The antenna becomes very broadband with 800Ohm feedpoint resistance. RFS feed their antenna with open wire feeder of 800Ohm, however you can easily feed it also with coax, whith a proper impedance transformer at the feedpoint to transform from 50Ohm or 75Ohm coax to the 800Ohm antenna impedance.
- If you choose to terminate the both sloping wires individually at the end the resistors at each end should be 400Ohm. The natural feedpoint impedance keeps 800 Ohm resistive - so no change in feeding the antenna
- RFS uses 100m sloping wires with the ST330. They also have a smaller version with 2x80m wires . My simulations show, that there seem to be an optimal angle, which provides smoothest pattern (no sidelobes, best F/B ratio, best match to resistive 800Ohm). As with 100m wires, the center mast is 22m, if you choose shorter wires, you should lower the center mast (!!) to achieve best pattern! The center tower of the smaller RFS antenna version is therefore only 16m high.
- The longer you make the wires, the lower the frequency becomes, where you can achieve gain and significant directivity.
- The terminated V-beams are so far the most interesting RX-antennas, as they offer gain (high bands, where you really need as much sensitivity possible!), high natural bandwidth (SWR better than 1:2 from 1Mhz to 30Mhz!), and easy installation and cheap cost. It can be easily scaled down to much smaller dimensions, as long you can manageat least 1.5wavelength per leg. The more the better. And also my practical antenna (see below) is almost as quit as the 170m Beverages, but with much better performance and higher sensitivity on the high bands.
- Since yesterday I have build a much smaller version of terminated V-beam: 2x41m wires, 60deg horizontal angle, sloping down from only 11m (in a tree) to ground level, 400Ohm resistors to a single ground rod. Feedpoint with very small BN 202-73 (8turns tot the ant wires, 2 turns to 50Ohm coax). The main direction is to 120deg from here (middle east, indian ocean, east africa).
- My new V-beam is also on use for WSPR reception used with wsprdaemon script from Rob
- here is the URL to check my test antenna out:
http://on5kq.ddns.net:8073
Ulli, ON5KQ
Recently I came accross the interesting information about Davis Station on Antarctica (VK0JLX)
https://www.qrz.com/db/VK0JLX
They are using commercial antennas made by RFS
The receiving antennas are ST-330s made by the company. At VK0JLS they call it sloping triangles, but essentially the antennas are terminated V-beams (2 longwires) sloping down from a center mast.
I found that interesting enough to do some simulation work with NEC2.
My findings:
- It seems to me, that the low horizontal wire which connects the ends of the sloping wires from the mast (with the ST-330 they are 100m long) are functionally NOT necessary. But if it is difficult to make good and stable earth connections to terminate the sloping wires where they touch the ground level, the horizontal wire is a proper way to terminate, without any earth connection.
- The antenna has a very good low angle pattern, not many side lobes and seem to be rather similar as a large log periodic beam. It sounds very quit and is therefore excellent for reception. Although you can use it for transmit as well, the efficiency is not the best (due to large bandwidth and losses in the termination resistors). However for reception it is hardly necessary to use a pre-amp. Nor is it even possible to use a pre-amp here in Europe, because broadcast stations from 15/17Mhz band are extremely loud - you can check it out on my online kiwi (see URL below)
- >15Mhz vertical radiation angles are around 10deg or even lower!
- The termination resistor with the RFS original design of their product ST330 is about 800Ohm in the middle of the horizontal wire which connects the ends of the sloping wire. This is also the feedpoint resistance. The antenna becomes very broadband with 800Ohm feedpoint resistance. RFS feed their antenna with open wire feeder of 800Ohm, however you can easily feed it also with coax, whith a proper impedance transformer at the feedpoint to transform from 50Ohm or 75Ohm coax to the 800Ohm antenna impedance.
- If you choose to terminate the both sloping wires individually at the end the resistors at each end should be 400Ohm. The natural feedpoint impedance keeps 800 Ohm resistive - so no change in feeding the antenna
- RFS uses 100m sloping wires with the ST330. They also have a smaller version with 2x80m wires . My simulations show, that there seem to be an optimal angle, which provides smoothest pattern (no sidelobes, best F/B ratio, best match to resistive 800Ohm). As with 100m wires, the center mast is 22m, if you choose shorter wires, you should lower the center mast (!!) to achieve best pattern! The center tower of the smaller RFS antenna version is therefore only 16m high.
- The longer you make the wires, the lower the frequency becomes, where you can achieve gain and significant directivity.
- The terminated V-beams are so far the most interesting RX-antennas, as they offer gain (high bands, where you really need as much sensitivity possible!), high natural bandwidth (SWR better than 1:2 from 1Mhz to 30Mhz!), and easy installation and cheap cost. It can be easily scaled down to much smaller dimensions, as long you can manageat least 1.5wavelength per leg. The more the better. And also my practical antenna (see below) is almost as quit as the 170m Beverages, but with much better performance and higher sensitivity on the high bands.
- Since yesterday I have build a much smaller version of terminated V-beam: 2x41m wires, 60deg horizontal angle, sloping down from only 11m (in a tree) to ground level, 400Ohm resistors to a single ground rod. Feedpoint with very small BN 202-73 (8turns tot the ant wires, 2 turns to 50Ohm coax). The main direction is to 120deg from here (middle east, indian ocean, east africa).
- My new V-beam is also on use for WSPR reception used with wsprdaemon script from Rob
- here is the URL to check my test antenna out:
http://on5kq.ddns.net:8073
Ulli, ON5KQ
Comments
I have now placed a 16db pre-amp at the feedpoint of the V-beam antenna for frequencies above 10Mhz. The amplification slopes up, when frequency is higher.
10Mhz and lower = no pre-amp
gain of pre-amp on 14MHz: +8.5dB // gain of V-beam without pre-amp = 1.8dBi (due to low efficiency)
gain of pre-amp on 18Mhz: +15.6dB // gain of V-beam without pre-amp = 4.1dBi (low efficiency)
gain of pre-amp on 21Mhz: +15.9dB // gain of V-beam without pre-amp = 5.3dBi (low efficiency)
gain of pre-amp on 25Mhz: +16.2dB // gain of V-beam without pre-amp = 6.7dBi (low efficiency)
gain of pre-amp on 28Mhz: +16.6dB // gain of V-beam without pre-amp = 7.4dBi (low efficiency)
The noise figure of the pre-amp is approx. 5db.
Probably I need some attenuation at the rx-input, when Radio Rumania switch on their powerhouse (500kW)... or in the evening.
The antenna system is now very sensitive on the highbands, probably too much in the evening... we will see...
All the figures are taking into account for the noise graphs generated from wsprdaemon script....
That is why the band noise on 10m is now measured extremely low arround -168dbm/Hz
(actually I wonder, if this figure really is correct, but I have no better data at the moment)
An amateur radio friend of mine was with the British Antarctic survey many years ago, and was on the ice for a while.
I think the reason they use terminating wires on the V beams in Antarctica is that the ice & snow surface is more like a dielectric than traditional 'ground'.
In fact I've heard of wire antennas being laid directly on the ice which seemed to perform OK.
Apparently HF propagation is really weird around the poles.
Regards,
Martin - G8JNJ
However Davis station seem to be not on top of 300m thick ice, but the foto's on qrz.com indicate, that there is even accessible soil without ice at all (during summer).
However it looks very rocky, so to drill and install stable earth connection is at least extra work. So the way how RFS terminate the sloping v-shap longwires is a nice way. It also helps to keep the antenna-currents balanced and the radiation pattern clean.
However if you have stable earth connection available the 400Ohm loads directly to ground at the end of the wires work at least as well than without earth connection the RFS way... At least I had never thought about the way RFS does it. So it was worthwhile to mention here, I think.
Radio Rumania almost broke my receiver this afternoon with the V-beam directly on them... it was necessary to add a 6dB att, at the Kiwi rx-input. I do not want to place a notch, as I generally want to keep almost constant noisefloor levels.
After the first day, I am really happy with this new antenna, also because I can place it very easily in my own property. I do not have any towers and only very few trees... so not easy to get low angle radiation on the high bands with horizontal pol. antennas...
My old tower with the horizontal LZ1AQ loops (it was excellent as well!) is unfortunately down, because of safety issues with the too light/old construction. Too much work to rebuild...
Ulli
that's very correct, but for antennas purely for reception the pattern shape is much more important than gain figure!
Below 10Mhz with my 2x41m terminated V-beam there is not gain (over isotrop radiator!) at all.
40...160m are as inefficient as a Beverage antenna - so reaching -15dbi on 160m ! I have taken care of that in the noise graphs for WSPR. If you consider, that a simple horizontal dipole at halfwave height over good ground reaches +8dbi (due to ground reflection)..... the V-beam is very inefficient indeed!
However look at the pattern!
Very good for reception for DX - low angle ! Also the pattern is rather clean without big side lobes. I found also very good F/B ratio. The US- signals on the Beverage to USA on the high bands are at least 20db stronger, than on the V-beam to the opposite. And what the V-beam hears from Africa you can not even hear a whisper on the Beverage... So highly directive on 20m and up, even with only 2x41m - surprising.
Also the characteristic of rapid dropping efficiency towards lower frequencies while maintaining a bit better the gain towards higher frequencies is what I really like. That is because it is much more easy to boost efficiency with an additional pre-amp.
If the stations become too loud on the low bands in the evening (lower Shortwave BC-bands and MW and LW) it is almost impossible to use a pre-amp for 10m, where you really need more sensitivity without a lot of filtering...
My Conclusion: it is a very simple and useful broadband antenna to get to work nicely...
That's why I mentioned it here.
Ulli
Notes related to Loop on Ground antenna running in another thread.
http://forum.kiwisdr.com/discussion/comment/9446/#Comment_9446
I am planning exactly, what you describe to make in coming summer. However my center mast will be only 12m... (strong fiber pole)
I remeasured the wire length - it turned out, that the wires are even a bit shorter - so instead of 41m only 38m long. May be with the old direction it was not possible to put 41m wires on own property.
This morning I measured SWR very precisely with my DG8SAQ - VNA.
Check the attached document with the curve and data per band.
It is amazing that with even a gain antenna (above 10Mhz) the SWR is nowhere higher than 1:1.5, except 160m, which is still below 1:3 - the antenna is really broadband and not at all a dummyload.
Why I prefer good SWR even on purely RX-antennas ?
Because stable non reactance Feedlines are very easy to handle, with low loss and much less chance of Noise ingress.
In my current fieldday test set-up I do not use a current balun in addition to the step-up transformer at the feedpoint. An additional BN202-73 with 5-6 turns of twisted enamelled copper wire (0.28mm is almost exactly 50Ohm twisted pair!) should kill any common-mode current on the feedline...
I am sorry for more noise on the new direction of the V-beam. It fires directly towards my neighbors, who use a lot of noisy stuff...
Edit: I got error, when I tried to upload .png file from the VNA. then I got the double posts. Will try to upload the picture in seperate post .
Attachments:
https://forum.kiwisdr.com/uploads/Uploader/38/9b7f3d2b248a50077a0e1428ee92ce.png
A wonderfully quiet location in comparison to my own sites, though it would appear someone in your vicinity has powerline ethernet.
I will try to replace the antennas 300m further out on the farm fields, which I may use during the winter. However from mid march the agricultural work begins already, so this will be for next season. The current V-antenna is on my own property, but beaming directly to my neighbors. My first test with this antenna was towards Africa (south east, which was much more quit with PLC noise down by 15db (as the NEC2 simulation also suggested for the Front/Side ratio)
In the SWR picture above, note the very effective "natural" MW-BC filter of the antenna. With a length of approximately 2x 40m the V-beam becomes an ordinary dipole on 160m with low impedance, so SWR is already not good anymore. Reducing frequency the SWR drastically rises and there is a huge mismatch error on the lower frequencies, which helps to reduce the very strong BC-stations during night times here in Europe. Additional MW-highpass filters are NOT necessary with this antenna. You need a different antenna for MW and lower in frequency.
"....Do you have a sketch / drawing of the wiring and connections? ..."
Does this help ?
At the feedpoint you need a step up transformer and also I would install a current choke - for receive only this can be very small!
So looking from the antenna to the receiver:
- From feedpoint to high impedance side of the step-up transformer - further to one side of the current choker - other sider of choke to 50Ohm coax feeding the receiver...
If you use 50Ohm Coax cable this is what I have taken:
Step up transformer:
Amidon BN202-73 core
0.28mm enamelled copper wire, 8turns to the antenna, 2turns at the 50Ohm side
Current choke:
Amidon BN202-73 core
0.28mm enamelled copper wire, twist the wire closely together - this forms almost exactly a 50Ohm twisted pair line. Then 6 or 7 turns on the BN202-73 core one side to low impedance side of step-up transformer and other sider to coax.
The Choke Z is about 5000Ohms for any common-mode currents and less than 1db attenuation for the desired differential mode (signal) current.
The antenna will work from 1.8... 30Mhz... not yet tested on 50Mhz, if it is useful, as there are no condx...
Attachments:
https://forum.kiwisdr.com/uploads/Uploader/8f/aea9010097c69cc2595a654cde60d6.bmp
if you want to try the antenna, I would recommend to plan two of them - so you can switch direction and find out the behavior, which is very useful for judgement.
I know you also listen a lot outside the Ham-bands, like me. (SWL). Today during exceptional bad condx (G1 Storm), I compared many of the kiwi-SDR tuning to KTWR station from Agana,Guam. Although I have lots of PLC-noise from my neighbors, there was not a single Kiwi which I could find, who could copy the station in the 25m band and later the day in the 31m as loud as on the V-beam. So my guess is, that certainly the antenna has considerable directivity. But due to the PLC, it was not a pleasure to follow the program. but is was possible, even while KTWR was weak at the time. Other Kiwis could not even understand the program, just to weak the signal was, even in totally quit kiwi locations!
The station was by far louder on the V-beam through the PLC than any other Kiwi from central Europe I tested... (I checked only the kiwis, I know with a very quit location)
It is a pitty, that to real find out the potential of the antenna you should have a better noise-free location than me.
So I want to encourage to build the antenna carefully in your rural location with a good reference for comparison.
I could not carefully test (or even measure!) the directivity yet. Using WSPR, it indicates about 20db difference at low angles on 20m (Front to back. when comparing it with my beverages and my vertical dipole). But it would be much more reliable to make A/B comparisons of the same antenna in different directions... so far I could not do it.
During the next couple of month, I will try to find a better location, where I might be able to set-up the kiwis in the future for less PLC-noise...
Ulli