Over the last few weeks I have been trying to correlate the results I have achieved with three different antennas, an outside Turnstile, a loft-mounted QFH, and an outside QFH from Paul Hayes, to the characteristics of the antenna. (See here for more about QFH antennas.) To do this, I have used instrumented versions of both my WXtrack orbit prediction program and SatSignal image decoder program. By combining these two in my NOAAplot program, I am able to plot the signal-to-noise ratio (SNR) versus the satellite observed position. You get a lot of information, and it’s really three dimensional, so rather difficult to visualise. By turning the SNR into colour, and plotting versus observed position, the results below are obtained. I found that the outside QFH gives too much pages interference to be directly connected to the RX2 receiver, so I have added a small filter and this is my current working configuration. (The filter acts as much by providing about 1.5dB insertion loss as by removing the pager signals).
[Technical discussion: I measured the RMS value of noise in the channel-B (sensor channel 4) “look-at-space” signal compared with the peak white value of the signal. I would expect a maximum value for the video SNR of around (256 / 0.29) or 59dB simply from the quantisation due the 8-bit digitising used for the signal. Note that because of the FM demodulation, there would be a further factor to take into account to get the channel SNR compared to the video SNR. I haven’t done that as yet.]
In the plots below, white is noise free, yellow and green show slight noise, and the blue has noticeable noise. Thus you should be able to get a quick judgment of an antenna’s performance by the radius of total brightness visible, if you see what you mean…. The circles are at 500km intervals, thus the plot is 7000km square.
Turnstile polar response
Loft-QFH polar response
Outside QFH polar response
Outside (QFH with filter) polar response
Note that while the loft-mounted QFH has a more limited coverage, it is more uniform over the area where good signals are received. The outside QFH gives by far the best result, but with the filter gives poorer noise performance. Note that the radial elevation scale is against distance rather than elevation angle to emphasise the performance at low angles. It is not a mistake that some of the tracks appear to cross, as I have included both NOAA-14 and NOAA-15, which have different orbits, in the results.
The next step was to try and summarise the performance so that the differences were more clearly visible. The first attempt was to average the SNR versus elevation angle. A weighted average was used to give more prominence to the better SNR values. Part of the justification for this is that on NOAA 14 I do suffer pager interference (at least with the outdoor turnstile), which artificially depress the SNR. You can see the results below.
Finally, I realised that it would mean more in terms of amount of quality picture if the elevation angle was replaced by the distance of the sub-satellite ground point. As you can see, this places more emphasis on the low-angle performance.
My current conclusions are that at certain angles the QFH does indeed perform a little better than the turnstile, but that the loft mounting is detracting from its performance, so better get a QFH outside as soon as possible! The loft-mounting introduces both a flat signal loss, probably worse if the roof is wet, and screening by the other objects in the loft. The outside QFH (at least at this location) requires extra filtering to prevent pagers from completely ruining the pictures.
What I had hoped to see, but cannot clearly identify, are if the turnstile has any well-defined nulls. I certainly see these on the pictures I get, but identifying them in 3-D space is not easy. I also compared the averaged SNR against azimuth, hoping to see which directions were good and which bad from this location, but there is nothing definite that I feel I can yet conclude from the data I have so far. See what you think…
It is difficult to see the detail on this graph, and to draw conclusions. However, it is clear that the outside QFH is best, with the QFH/filter combination being perhaps 5dB worse. Towards the north-east, the loft QFH seems as good as anything, so presumably I have a clear take-off in that direction. Everything seems to perform more similarly to the north and south, however this could be because there are no low-angle passes across the north and south, as there are to east and west, so there is much less continuity of data.
Source: satsignal.eu by David Taylor.
All round tubing elements were cut with a plumber’s tubing cutter available at most hardware stores. The aluminum boom and rod, and the teflon can be cut with a sharp hacksaw.
All aluminum that I used is T6 hardness.
I purchased my aluminum at Ridalco, 1551 Michael Street (near St.Laurent and Belfast). Total material cost was under $25 (incl tax), but I am a real shopper and we were building 4 at a time.
The dimensions that I used are:
Because all of the elements of this antenna, including the driven element, are fastened to the boom (metal to metal) the elements are at ground potential and tend to have a reasonable immunity to lightning damage. Also, when measuring this antenna with a simple voltmeter, you should see a very low resistance short circuit.
This antenna is short and light, and so you can choose to centre-mount it or end-mount it. You can use exactly the same boom-to-mast mounting arrangement for either mounting. If you end-mount, as I did, be sure to leave enough square boom at the reflector end. I’d suggest a little over 200mm spacing between the reflector and any mast pipe or tower leg (that way, the mast or tower acts as a 2nd reflector when positioned for vertical polarization (my theory anyway). Allowing 963mm for the boom and an additional 204mm (abt 8″) plus 50mm (for mtg). Allowing for a bit of an extension to hang a counter balance on, I ended up using an overall boom length of 5 feet. You could use a bit less if you prefer. When you are intending to end mount, keep in mind that the reflector goes nearest the mast or tower, so start measuring from the director end.
I used 3/4″ square tubing for the boom. A feature of 3/4″ square tubing that you may find useful is that it slides just perfectly into a 1″ square tube. I attached my mount directly to the boom.
Alternately you could fasten your mount to a 10″ piece of 1″ square tube. By sliding the 3/4″ into the 1″, then putting a bolt through the 2 square tubes, you end up with a handy end-mount arrangement which allows you to quickly (manually) change from horizontal to vertical polarization. Caution: Do Not Drill the 2 pieces while they are inserted one into the other, or you will never separate them. Drill the big one first, then mark the small one while inserted into the big one. Separate them, then drill the small one.
The elements are 1/2″ round tubing and are fastened to the boom by attaching to 2″ square plates using pop rivets..
The details of the driven element are:
Note that all elements (driven and parasitic) fasten to the same side of the boom.
This antenna requires a balun to feed the two sides of the dipole. To make the balun, start with a piece of RG58A/U (stranded centre conductor is preferred if you can find some) which is 500mm long. On each end, remove about 1″ of sheath, then either pull the centre conductor thru the exposed braid or unweave the braid. Attach an eye terminal to each end of the centre conductor. Then cut each end’s braid so it is about 1/4″ long. Now attach a terminal eye to each end of the braid. Be sure to solder all terminals, but be careful not to melt the insulation. You can change the length of the braid slightly to suit your antenna. The centre conductor of the balun connects between the 2 screws that hold the 2 rods. Each end of the shield is grounded to the screw that holds the insulated block to the boom. The balun and feedline extend along the boom towards the reflector end (assuming that you are end mounting).
When fastening the rods, first flatten them slightly with a hammer and anvil, then punch them and drill them very carefully.
Note that the 50 ohm coax (213) feedline centre conductor connects to either one of the screws which hold the two rods to the teflon block. The shield of the feedline is grounded to the screw that holds the insulated block to the boom. Use terminal eyes. I usually lightly crimp the terminal, then solder it with a heavy duty soldering gun or a small propane torch. If you wish, you can strengthen the connection by using sealing heat shrink tubing.
I recommend sealing all connections with liquid rubber available from electrical contractor supply houses. Its expensive but worth it.
When adjusting this antenna, you may use either an antenna analyzer (if you can find one for 222MHz), or a radio/xvrtr and a good SWR meter. You will have to experiment with the sliders on the driven element while watching the SWR. I like to use my FT-817 with my xvrtr to do this. I make several test transmissions at different frequencies while recording the SWR reading. It is important to find the resonant frequency of the antenna. After you have found the point where the SWR is best, adjust the sliders in or out, to get the best match. Now go back and confirm the resonant frequency. At this point, you may trim the driven element if the resonant frequency is too low. I ended up with my sliders exactly 320mm apart on their inside edge. Assuming you have followed these instructions, yours should be very similar.
According to my tests the antenna was centred at about 222.200 MHz and was better than 1.5:1 up to 223MHz. I have found the design to be reliably repeatable.
This antenna was used in the June 2001 VHF contest and worked very well, in spite of the fact that it was at rooftop height. The antenna was mounted on the tower mast at about the 51′ mark in October 2001. It performs exceptionally well, and its hard to believe that it is only 6 elements.
Presented here is an experimental LPA for the Australian UHF broadcast TV bands IV and V. They span about 520 MHz to 820 MHz. The original design was started when a friend wanted to build a cheap and effective TV antenna for his girlfriend’s TV.
The boom/feeder is constructed out of PCB material, two strips of single sided fibreglass substrate, with about ‘1 ounce’ copper cladding. The material is available from DSE in 300mm square sheets, so one fundamental constraint on the design was a 300mm boom length.
This constraint puts Tau at about 0.8 and sigma at 0.11, giving approximately 6 dBi of gain and a fairly flat VSWR profile across the design range. Originally the PCB/transmission line was carefully designed for 75 Ohms, but the dimensions required were mechanically challenging owing to the high dielectric constant of the substrate. 20mm wide strips, glued back-to-back was the final choice, mainly for its excellent mechanical properties rather than any real Zo design point.
Here you can see the boom marked with element spacings and reminders about the phasing of alternating elements. The two largest elements are also seen, like all the elements, about 20mm extra length was cut, and trimmed as the final construction step. The boom is already laminated at this point, common super-glue was used for this step.
|Element||Tip-Tip Length (mm)||Spacing (mm to next element)|
Eutectic Tin/Silver solder was used, again not for any particular design purpose, it was what was plentiful and on hand. The higher melting point of this alloy compared to normal Tin/Lead solder made holding the shorter elements while soldering them to the boom quite a painful experience. The elements themselves are 1.5mm diameter solid copper wire, from the centre of heavy-duty mains cable.
The array is fed directly by 50 Ohm coax (DSE branded RG-58CU in this case). The feed point is as the ‘sharp’ end of the array, the braid simply going to one side and the centre conductor to the other. A sleeve bead choke of VHF/UHF ferrites helps maintain balance, as does an air-wound choke coil in the coax. Some experimentation was performed with the placement of the sleeve choke, the final decision was to take the coax along the braid connected side of the boom/feeder and place the chokes beyond the end of the boom. There is no terminating stub. This arrangement produced the cleanest pattern
A short piece of wood dowel extends the boom and provides a mounting surface. Nylon cable ties are used to hold the feeder/boom to the dowel and dress the coax from the feed-point back along the boom.
Performance testing is work in progress. So far the antenna shows excellent balance for such a simple feed. The first null is outstandingly deep (full) and symmetric off either side of the main 60-90 degree lobe. Polarization purity is also excellent, with full nulls being achieved by cross polarization. Tests have were carried out by ear, eye and S-meter, using the local North Head TV translator as a signal source. The antenna appears ‘useful’ well above and below the design range. No transmitting tests have been performed yet.
Once the feed VSWR has been assessed a new prototype will be constructed out of hobby store brass tube and box stock. The pattern and bandwidth seem excellent so far, so only the matching really needs validation/tuning. The extra expense of the brass material may not be worthwhile, the cheap (but somewhat easily bent) wire is doing an excellent job.