Progress update:
Well - I’ve tried a few things.
KiSS - simple and rugged, a near-end linked EFHW
I ran 60m over this week’s activations using ZL1THH’s approach of a near-end linked EF60mHW with movable feed point for 40m. Rugged and resilient and promising. An additional link would give me 30m full-wave.
A shorter solution: adding 60m and 30m to an EF40mHW
A look at the Portable 7-Band EFHW Antenna gave some confusing hints that this approach might (or might not) cover both 60m and 30m. Figure 32 shows the 2nd VSWR dip where my simulations put it at 9.5MHz, but Figure 41 shows it bang-on 10MHz - much more useful.
However, construction of the EF40mHW with 25uH loading coil at 15.785m showed the simulations to be bang-on with the 2nd VSWR dip at 9.5MHz. Varying coil inductance vs position could bring that 2nd VSWR dip lower in frequency, but I never got it near enough to 10MHz to be useful. In the end, wind stopped play.
I’d be very keen on details from anyone who’s managed to get this approach to tune on 60m and 30m, as-per figure 41.
Not an ideal approach due to a loading-coil in the second half of the antenna (difficult to deploy / switch in-out / retrieve in scrubby country). But I’d be keen to play with it.
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Seeking a 60m/30m/??m coil-shortened EFHW.
I’ve lost much hair tussling with 4NEC2 and EZNEC - the dreaded ‘Error 6’ (seems to be something to do with poor handling of VB floating-point arithmetic when running under Wine) leading to me going as far as setting up a MS-Windows VM in an attempt to get it to run error-free. Desperate times indeed.
Then, I discovered that if I hit F12 when running Rob / DM1CM’s excellent online EFHW designer I can override his default selection of inductance, length values and get that to simulate all coil values, coil positions and antenna lengths. Speedy progress ensued.
The designer suggested a 25.675m wire with a 10uH coil 2.5m from the feed point would give me 60m, 30m and 20m.
A build of that indicates it to be viable. I don’t have means of measuring inductance, and had to remove about 30% of the turns off my coil to get things to line up - so I’ll have to test the actual inductance used at a later date. Not clear if my calculation of inductance required was out, or I got the turns required to achieve that wrong.
On the VNA I get VSWR minima at 5.3MHZ, 10.1MHz and 14.3MHz. All three with nil or near-nil complex impedance component. Handling the cable or connectors has no impact on those minima indicating that they are genuine and should be reliable.
Connected to the TX-500 all three show VSWR or 1 or 1.1 when set up in an inverted V at 1m-5m-1m on the SOTApole on flat ground at home.
Looking forward to trying it out in the field to see if it really works in a range of conditions / deployments!