(Changes is in bold text.)
I have tried to make a coil to an tunable vertical antenna for portable use. I made two different coils, one 40 mm diameter and one with a dia. 32 mm. In a simple first test it seems to be no difference between them. The coil sits in the bottom of the structure on a 70 cm long aluminum tube of 10 mm diameter, and provided above the bobbin with a 3 meter telescopic rod. With the rod fully extended, I can with the slide switch on the bobbin Meeting of from 80 m to 20 meter band. Then I remove the coil and the meeting of 17-10 meters directly to the bottom part + telescopic rod. From 6 meters to 2 meters, I use only teleskopic rod and agree with it. For this purpose I had to make a coupling sleeve with the M8 nut on one end and UNF 3/8 ”on the other end with a stub copper tubes between. UNF 3/8” is the measure Buddipole have at the foot on its telescopic rod, both at 1.7 meter and 3-meter rods.
How do you do this coil? It is simple. A piece of 40 mm or 32 mm white sewer pipe to use as bobbin, length depends on the wire you use. I had 1 mm ”magnetic wire” lacquered copper wire, about 13 m long. Expect 85 laps, multiply the diameter of the wire with paint on. Then add about 10% of the spool and you have the coil length. Coil body should be at least 3 cm longer at each end. If you want a contact on the bottom of the coil add the contact width in addition to these 3 cm. with 32 mm coil body becomes a bit longer, about 100 turns.
The tuning ring is made of a pipe one dimension larger than the spool body, inside the tube (about 3,5 cm long) i mounted a copper strip in by folding the ends around the edge of the plastic tube. At this edge, you can solder a soft cable which will then be mounted in the bottom of the coil. Try as the ring is on the outside of the coil, it is very sluggish, take a pen hammer and tapping into a rounded shape for that tube to evade the contact rail easier to the coil.
I dressed this ring with a foam sheet 3-4 mm thick to get the distance to the coil winding and do not affect the tuning ring so much with handcapacity.( piece of sleeping pad or similar is perfect). still, there is som hand capacity you cant avoid.
(The images in Swedish page shows how I arranged the installation of the antenna, on the coils is no connectors except the nuts on the ends, connector is in the bottom of the lower 10 mm tube is BNC chassie connector mounted. )
I have also added a bottom pipe in the coil of about 70 cm length 10 mm alu. With this, antenna was a bit easier to tune. Also, I have made two cable trunks containing the cut cables for ground plane.
A balun 1:1 at the connection yielded that the SWR is now some bands completely gone and that there will not be any impact when moving at the station, which could happen earlier. The connector and strain relief sits on a copper clamp meant for copperpipe fastened with two screws in the base insulator.
Testing the coils.
Now I have the opportunity to try the two coils together at the same time. They all work the same, so technically no difference. But the narrower but longer became lighter and less bulky to pack, so it becomes, I’ll continue to use as a portable vertical forward. I have not measured any q values or the other, but only tried purely Traffic. The naked copper surface may be required trim sometimes to get a perfect connection.
To tune this antenna just slide the ring up and down for highest signal level, then fine tuning with radio on low power and swr meter for lowest SWR. Hand capacity can occure but you will find a good methode to do this tuning.
Have you built this antenna and found that it can be difficult to get contact?
I have found in recent tests with the VA-3 antenna that the coil seems to ”eat” part of the effect. Do this: Where the cable to the tuner ring are connected to the coil end make you a break on the spool. drilling a 2 mm hole about 2 cm from the junction next to the copper wire in the bobbin. Cut the copper wire directly on the connection point and insert it in the hole. Tighten the thread tightly and glue in the hole. Now you have an antenna in which only the portion of the coil that is effective is included in the circuit. With such I have driven all over Sweden, SM2 to SM7, but not SM0 and SM1. (Not awake?) In contrast, ES and DL was good here. This with 5 W CW portable from JO89tt