This area will include info on the new 8m and 5m band
Band Pass Filter for the 8m band
By Phil Rogister EI9KP
For experiments on the new 8m band (40MHz) including cross band activity 10m/8m and 8m/6m a requirement arose for a Band Pass Filter. My antennas are mounted on the gable wall and all around the house so a strong local RF field (out-of-band signals overloading a receiver’s front-end) poses a problem with this kind of set-up. The 10m, 8m and 6m antennas are configured in horizontal polarization, additionally, in close proximity are vertical antennas for 8m, 4m and 2m/70cm FM as well as an Inverted-V wire dipole for HF bands.
Commercial Band Pass Filters are available for HF up to 10m, 6m, 2m and 70cm but I rarely come across them at rallies, especially the 100W models. Tried and tested BPF designs for the 6m band (subsequently remodeled for the 4m band) are available on various websites1.
This experimental BPF for 8m is based on those designs. I initially ‘remodeled’ the filter not by using formulas but by extrapolating onto graphing paper, the curves representing capacitance and inductor values versus frequency. In the process I also traced component values for possible future BPF’s for the 9m (34MHz) and 5m (60MHz) bands.
For the capacitors I chose the nearest commercial values from the ATC 100B (500V) series.
Coils would have to be wound by hand on a rigid coil former of chosen diameter. Online programs are available to calculate “single layer air-wound coils” inputting whatever wire and core diameter you may have. A wire length of around 300mm is all you per coil used in this BPF, leaving about 20mm for connecting to the N-socket and copper board. The actual lead length should be trimmed according to box size and chosen board layout.
I used a free simulation software called “RFSim99” to run an order- of-magnitude check of the Band Pass Filter [curve] from the obtained capacitors and inductor values.
Having only 1.40mm2 enamel Cu wire in my box of spares (a different wire diameter from the original article) the coils needed a few attempts to get to the correct inductance to center the filter’s pass band. The air would coils have a low inductance, typically 200nH … 500nH, my LCR meter could not measure them directly.
Initially the coils for the 8m BPF were “too long” however a typical pass band around 34MHz was visible on the analyser and coil dimensions were duly noted for a future BPF. Taking 2 turns off brought the pass band nearer to 40MHz. Furthyer tweaking by stretching or compressing the coils brought the pass band to the desired 40…42Mhz.
Inline loss and SWR (with a 50Ohm coaxial termination) were measured using a network analyser, all within expectations: loss <0.50dB and SWR <1.20 in the pass band.
Of concern to anyone planning to operate on 8m band frequencies would be the third harmonic which falls within the civil VHF Air band. For this BPF suppression at 120MHz was measured at -48dB. This should be read in addition to filtering by (usually) a Low Pass Filter built into the transceiver.
Whilst I did not make specific power measurements with given component values (especially using 500V ceramic capacitors) the 50W power level permitted to EI stations should not pose a problem.
With a couple of hours of focus this “kitchen table” project was made for a component cost of around 30 Euro. For component values, electrical diagram, construction details and measurements please have a look at the information below.
I hope you will be encouraged having a go at building a BPF for a chosen Low VHF frequency and wish you every success.
73 de Phil EI9KP
1 Article (printed) by YU1LM, The 4m Forum (http://www.70mhz.org/).
List of materials:
Box: aluminium, screw on lid, with gasket, external dimensions 125x80x55mm. A smaller model 115x65x55mm would have suitable as smaller diameter wire was used for the coils.
Connectors: N-chassis connector with 4xM3 screws
Capacitors: ATC 100B series 500V
Coil wire: enamel Cu 1.40mm2
Single sided copper board, dimensions 115x75mm for inside box. Off-cuts used as horizontal strip 70x15mm and “solder islands” 15x15mm.
Radio update for use on 8 meter band
The following transceivers have now been tested for use on the band
Icom IC 706, IC 7200, IC 7300
Yaesu FT 817, Ft 857
Vertex Ftl 1011h
Any of the above that have been wide banded will work on the new allocation. Although these some of these radios will seem to tune to the new 5 meter 54MHz allocation, none will operate successfully. The only sensible option for this band is a transverter.
Also worth noting that if the current used when operating out of band is very high against the power output there may be a problem filtering and the operating on that frequency.
P.S. The use of radios on this band is at the owner’s risk.
From Phil EI9KP
Delta Loop construction
For a test set-up the fiber mast was adequate, however, it does not like wet & windy conditions as the fiber sections will collapse even when taped. For a more permanent set-up I plan to use a GRP (glass fiber reinforced plastic) tube, reinforcing the lower end with an aluminium tube, to mount in a wall bracket. The horizontal wire support is 20mm PVC electrical conduit, the loop wire running inside. I am using a similar construction for the 6m and 4m SOTA Delta Loops. For portable use the support comes in two sections but for permanent use there is no reason to do that. I will use some self tapping screws to stop it spinning around under windy conditions. The loop wire is surplus military field telephone wire, a very thin diameter twisted pair type.
The impedance adapter is a quarter wavelength 75 Ohm coax (RG-59), to achieve a 50 Ohm feed. The length is 1.30m end-to-end; I strip back 1.50cm either end to accommodate mounting the cable on a 50mm tube, a recycled “silicon sealer” tube. I tightly wound the cable around the tube, pushed it through 6mm holes, one end soldered onto an N-chassis connector, on the other end I soldered cable shoes and fixed it to M6 bolts. The Delta Loop wire also has cable shoes fitted, I used wing nuts for practical portable use. It is kind of visible on the photo but I don’t have a good close-up.
The center plastic support is cut from 7mm kitchen cutting board, hi. The additional holes are for reusable tie-wraps, again for portable use.
The schematic and picture show the 8m Delta Loop in horizontal polarization, the feed point is at the bottom so it looks like an upside-down Delta. I also tried vertical polarization shifting the feed point to one side of the support and running the loop wire to the top of the supporting mast so it looks like a Delta. There was a little shift in resonance frequency, to be expected. The loop was usable for the 41.500 FM frequency, could be tweaked a little if only intended to be used for FM/ vertical polarization.
Use link below for images of equipment for the new bands.