Quarter-wave whip earth plane

I have one on order.

Possibly still worth a look. Rear door hinges are likely to survive a front collision. even a rear collision. Genuine parts are also an option.

On 3/15/25 10:23, Liz Tuddenham wrote: [Snip!]

I suppose it's enough to know that you want the curve to be as close as possible to the centre of the chart at the working frequency. That's where the load impedance matches the characteristic impedance.

A Smith chart is a chart of the voltage across a Wheatstone bridge with a funny grid mapping that back to the value of one of the bridge impedances. It was handy as a graphical aid to calculate impedance transforming networks and more, but nowadays computers can do that much better and much faster.

Jeroen Belleman

I've taken it out and cut the elements a bit shorter (they were too long to start with), I can get VSWRs well below 1.5 on 144 Mc/s and 70 Mc/s with two different elements. There is a lockable sliding joint so I can trim the length with the NanoVNA I have just bought. The tuning of the

I experimented with ways of increasing the capacitance to the van roof using aluminium cooking foil, but the difference it makes is not worth the bother of cutting a more substantial capacitor plate from sheet aluminium.

It has made a huge difference, I can see all my mistakes instead of blundering around in the dark.

On 15.3.2025 11.23, Liz Tuddenham wrote: I need to do a crash

The NanoVNA will show the reflection attenuation or VSWR if you're intimidated by the Smith chart.

The Smith chart shows the both the amplitude and the phase of the measured reflection. The coordinates are the resistive and reactive components of the reflection scaled to the system reference impedance (here: 50 ohms). The coordinate lines are circles for easy handling of the reflection. Traces of constant SWR are circles centered on the 50 ohm center point.

The VNA has very small coaxial connectors (SMA), so you'll need adapters to more conventional feed line connectors. I prefer adapters with a piece of cable between the connectors, to protect the small connectors from mechanical stresses.

Not so much 'intimidated' as 'outdated'. I passed an exam in 1969 which included them - and haven't used one since.

It gives me a huge amount of information which I am sure is exactly what I need if only I can get my brain around how it realtes to what I am doing with the hardware. I need to watch the trace on the screen and see what makes it rotate, expand, contract and turn back on itself so that I have a feel for how I can make it do what I want.

It is a bit like the description of learning to fly a helicopter: Waggle the stick about and see what happens - if you want that to happen, that is how you must waggle the stick.

It came with a set of short extension leads and I have connected a set of BNC adaptors to the far ends of those leads so as to remove any strain they would have imposed on the connectors on the side of the instrument. I have already standardised my R.F. interconnections with adaptors on each piece of equipment to convert them to 50-ohm BNC, so my learning curve shouldn't be complicated by intermittent connections from botched adaptors.

This evening I've also made up a couple of BNC to croc-clip adaptors for testing circuits on the bench. For tuned circuits I have an Edometer that ges up to 100 Mc/s, but the VNA will cover frequencies much higher than that.