Improving Earthing on a Magnetic Mount

A number of years ago I bought some classic "black stick" monoband HF mobile whips. These consist of a fibreglass section at the bottom onto which a wire has been helically wound. Some of the wire is wound with wide pitch and some with tight pitch to make up a loading coil. A top section is made from a springy steel whip which can be adjusted for length to bring the antenna to resonance on the desired frequency. I bought them in pairs with the intention of making loaded dipoles for portable operation. I have had some success with a 18Mhz dipole, but the 10Mhz and 3.5Mhz versions never worked very well and were always difficult to adjust to resonance.

These days when I go out for a portable session I normally take some 10m fishing rods with me. I use these either as supports for dipoles and loops or as a simple verticals. Although these take some time to erect it's worth it if I'm going to operate from a site for a few hours. During the colder months I've resurrected the mobile whips for use "static mobile" while operating from inside my warm car.

Previously I had tried a 19cm diameter mag-mount with these whips, but not unexpectedly that only worked well with the 18Mhz whip. On the lower frequencies there is insufficient capacitive coupling between the base of the mount and the car roof to provide a good earth. This results in high RF currents on the outer of the coax and RF on the case rig. While earthing the radio case to the car body removed the RF voltages, the RF currents were still present on the outer of the feeder. So I decided to try and provide a better earth with a direct connection between the car body and the base of the mag-mount.

I needed to find some way of making a good electrical connection to the car body and I found these convenient M8 bolts that attach the boot lid to the car body. They had sufficient exposed thread to take a large tag and an extra bolt. I soldered some "copper weave" wire onto the tags and added a screw terminal to the mag-mount base to allow quick connection and disconnection of the wires

This picture shows the copper weave heading off to the right and the black terminal used to connect it to the base. Also visible is a ferrite choke on the coax which discourages the RF current from using the coax outer as an additional path to the car body.

I've been using this configuration for a few weeks now and on 10Mhz and 18Mhz it works very well. I've observed 100s of mA of RF in each earth connection while only getting a few 10s of mA of RF on the coax outer braid. This has eliminated RF on the radio case which makes the antennas easier to tune as the resonant frequency is not changed by adding additional capacitance by touching the rig or antenna analyser case.

The pictures below show the currents flowing in the two earth connections (260mA and 240mA), the current in an connection between the radio case and the car body (45mA) and the current in the base of the antenna (660mA). This suggests that the capacitive coupling provided by the base is carrying 660mA - (240mA + 260mA + 45mA) = 115mA . These measurements were taken using the 10Mhz whip.

Double size mobile verticals

Although I have had some good results with the 10Mhz and 17Mhz whips, the 3.5Mhz whip has proved less effective. If you consider the next picture you might guess why !

The Red, Green and Blue lines show the lengths of the tightly wound coil sections on the 18Mhz (3cm) ,10Mhz (13cm) and 3.5Mhz (90cm)) antennas respectively. While the 18Mhz and 10Mhz whips are roughly centre loaded with a reasonable amount of high-current wide spaced helical wound wire below the loading coil, the 3.5Mhz whip is nearly all tightly wound coil. I wondered if the performance of the 3.5Mhz whip could be improved by providing a section of "un-wound" wire below the tightly wound section.

As mentioned above I have two antennas for each band, and because the same size of thread is used at both ends it is possible to screw two of the sections together. I did not want to destroy one by removing the existing windings, so I used a length of the copper weave to simply short out the existing windings.

Clearly this configuration is not suitable for use "on the move", it is strictly a stationary /P antenna.

A close look at the base mount shows the copper weave inserted between the thread of the whip and the mounting. The same technique is used at the top where the whip section screws into the top of the fibreglass section.

Obviously lengthening any antenna like this will have an effect on the tuning, but the results were different for each antenna. As expected with no change to the top whip section the resonant frequency of all three antennas dropped.

18Mhz result

With no adjustment to the top whip section the resonance dropped from 18Mhz to 15Mhz. Shortening the whip section restored resonance, but I also found that by lengthening it the resonance could be moved down into the 14Mhz band. This has allowed me to operate on 14Mhz, a band I've previously avoided as it is always full of contests at the weekends.

10Mhz result

The drop in frequency was not so great on 10Mhz only dropping down to 9Mhz. Since 10Mhz is one of my favourite bands for /P operating I decided to simply shorten the top whip section to restore resonance back into the 10Mhz band. I have experimented with a capacity hat just below the whip section to try and get the resonance down to 7Mhz. However it has proved to be rather unwieldy until I can get some lighter materials.

3.5Mhz result

Predictably the drop in frequency was much less on this antenna, only dropping a few 10s of kHz. This is not surprising as the extra 120cm section only amounts to about 1.5% of a wave length at 3.5Mhz compared to the 4% at 10Mhz and 7% and 18Mhz.

Better Match all round

What is noticeable is that the impedance at the base of all three antenna has increased. This results in a better match and lower SWR on 3.5Mhz, 10Mhz and 18Mhz plus the addition of 14Mhz as a usable band. On all bands at resonance the antennas can be used without the need for an ATU.

On air results

I've not yet had much time using these configurations but I have had good contacts on 14Mhz and the 3.5Mhz antenna has yielded some reasonable signal reports for the first time.

10th March 2016: 40m version added

On a trip to W&S earier this week I picked up the 40m version of these whips, and as expected it works well using the extension section. With the whip section fully extended resonance was just below the bottom of the band, and for operation around 7030 it needed shortening by 5 inches. This is not enough to bring the bottom of the whip close to tightly wound coil section so as yet I have not trimmed the whip to length (which could make it too short for use without the extension section).

Effects of Whip length and thickness

I have now acquired some 3/8" brake line adapters which allow the telescopic whip section from an ATX Walkabout antenna to be used at the top of the black stick antennas. I set up the extended 80m antenna on my metal shed roof and recorded the resonant frequency as I extended each section in turn. I did this twice, once extending the thin top sections first, and a second time extending the thick bottom sections first (see picture below)

The graph shows the expected result, for a particular length the fatter version provides more capacitance and hence a lower resonant frequency.

I think the slight kink in the green line is due to a section not quite being fully extended when I recorded the frequency, but it did fully extend when I pulled out the next section.

SWR Curves

I have used my miniVNApro to measure the SWR of these antennas:

New Bottom Section for Extended Whips

While plotting the SWR curves I noticed some odd effects to do with the copper weave used to short out the 80m coil in the bottom section. On 18Mhz the tuning seemed inconsistent, sometimes after adjusting the telescopic whip the resonant frequency changed in the wrong direction. I eventually traced this down to the exact positioning of the copper weave. The situation improved when I added some insulation tape around the top and bottom as it held the weave tight around the coil. However I still observed a strange "double dip" on 18Mhz which I think may be due to some parasitic resonance between the 80m coil and the shorting wire.

So today I've "bitten the bullet" and disassembled one of my two 80m sections. I unwound the loading coil onto a former so the wire can be reused later, and I replaced the crude riveted connection by drilling all the way through and using a bolt and nut and solder tag to attach a wire. This wire is not as thick as the copper weave I have been using, and it is a bit longer because it is helically wound up the rod.

Testing with the new bottom section

As expected the helically wound wire added some additional inductance over the straight weave used previously. This graph shows that with the telescopic whip set to the length that previously tuned the 80m whip to 3560kHz the new lower section drops the resonant frequency to below 3500kHz.

With the telescopic whip correctly adjusted there is ~33kHz where the SWR is better than 2:1.

SWR over 7000kHz to 7030kHz is better than 1.5:1

After adjustment of the telescopic whip the performance on 30m,20m and 17m were much as before. However the "double dip" seen on 17m has largely gone.