Tests for power on a 2m antenna

Caution! To avoid dangerous RF exposure, these tests are to be carried out using a maximum power of 5W only. A 2m HT is quite safe for the purpose.

The ability of a neon lamp to glow in the presence of an electromagnetic field made it a handy RF indicator of yesteryear. Many a handie talkie (HT) was tested by just holding a NE-2 near its whip antenna.

Curiosity led to a search of the junk box for a NE-2 but a different type, having plate electrodes, was found (see inset).

Power was applied and the antenna probed with the neon lamp. A static-charged plastic bag, placed in between , helped trigger the neon lamp.The ensuing red glow was sustained by the RF energy. The plastic bag may not be required when dry weather and synthetic clothes make it more conducive for the test.
Neon Lamp Test on J-Pole Antenna
At high voltage points the neon continued to glow even when moved away from the antenna.

Neon Lamp Test on Rubber Ducky
A more reliable method is to use a dipole as a pickup and a 6.3V-150mA dial lamp as the indicator.

My version of the pickup has two 21" telescopic whips fixed on a wooden strip, with the dial lamp soldered in between. It's quite convenient to push the telescopics in and fold them for storage.

Dipole-lamp pick-up - folded
With only 3.5W from my HT, and the dipole length adjusted to ½λ, the lamp glowed to more than full brilliance at a good distance from the antenna.

Dipole-lamp test on J-Pole antenna 
______________________________

'Wire Slim Jim' for 2m

This is a 'Wire Outside PVC' Slim Jim for 2m. It was made using enamelled copper wire and a vacuum cleaner nozzzle!

2m Wire Slim Jim
The wire ends and the BNC connector were fixed with self-tapping screws. The feed points were

connected to the BNC using the same wire. Water ingress points were sealed off with 'Araldite'.

An SWR of 1.5:1 was realized.

Related post: 2m J-Poles and a Slim Jim
______________________________

2m J-Poles and a Slim Jim

Last summer an attempt was made to homebrew a couple of 2m antennas, using parts salvaged from a disused VHF TV Yagi Beam.

The radiating element (folded dipole) provided the stub portion for two J-Poles. Bakelite pieces were used to ensure rigidity and also to locate the BNC connector.

2m J-poles and a Slim Jim
The moulded plastic box was used for the second version of the J-Pole. For the stub portion, the 2 elements were shorted inside the box. A BNC connector was located at the lower end of the box. A length of PVC pipe was fastened to the box to facilitate mounting of the antenna.

Close-up of the J-poles
'M-Seal’(epoxy sealant in dual component lump form) was used at the joints and to waterproof the connections.

The dimensions were arrived at using 'K4ABT's J-Pole Calculator'.

Absolutely no trials / adjustments were required to get the SWR very close to 1:1!

With the remaining material a Slim Jim came into being.

Construction was on similar lines and with good results.

Related post: Homebrew ¼ λ Magmount for 2m
______________________________

UHF on a VHF Rig

This circuit enables fair copy of strong 444 MHz signals, off a local repeater, using a 2m rig and an indoor 70cm ¼λ ground plane antenna.

UHF Converter 
The 300 MHz 5th harmonic of the local oscillator, mixing with the incoming signal, delivers output on 144 MHz.

UHF Converter - Schematic
The enclosure is a plastic trinket box reworked to fit two BNC sockets plus one RCA for the DC supply and also to screw the cover in place.

UHF Converter - inside view
The converse (VHF on a UHF Rig) is also possible by just changing over to a 2m antenna and a 70cm rig.
______________________________

A ¼ λ Ground Plane for 70 cm

It is easy to homebrew this ¼ λ ground plane antenna for 70cm. All one needs is a TNC connector (see inset), 1.6mm brass brazing rod and a pack of ‘M-Seal’(epoxy sealant in dual component lump form).

¼ λ Ground Plane for 70 cm
Radiating element length in inches is 2808/f MHz minus the projecting length of the TNC hot end.

Radial length in inches is 2950/f MHz plus a little for soldering on to the TNC connector flange.

The soldered points are sealed against water ingress using M-Seal.

Related post: Coax Collinear for 70 cm
______________________________

Fox Hunt Attenuator

Here's the circuit of my homebrew fox hunt attenuator.

Offset Attenuator - Schematic
It's a simple offset attenuator with a low part count.

The potentiometer is used to vary the signal strength of the attenuated signal, available at +1MHz, +2MHz, -1MHz, -2MHz of the fox frequency.

This attenuator was successsfully employed at a couple of fox hunts in Bangalore.
______________________________

Coupling a commercial VHF handheld to an external antenna

Some good commercial VHF handhelds have only one drawback - no external antenna connector.

Modification is also a problem as the rubber ducky is either integral or screwed on to a threaded stud, with no access to ground.

The result is a frustrated, repeater-bound ham. When a fellow ham in such a predicament set me pondering, realisation dawned that another rubber ducky could be used to couple the rig to an external antenna.

My own rig (which has a BNC connector!) was used to check it out. The coupling rubber ducky was mounted on the input of a homebrew SWR meter, with a dial lamp loading the output.

Rubber ducky coupling to a dial lamp

The glow of the dial lamp indicated a coupling efficiency of about 20% (2.5W to 0.5W).

A 2m external antenna was then connected instead of the dial lamp.

Rubber ducky coupling to an external antenna

On-the-air tests were successful. Comfortable simplex QSOs, not possible with the rubber ducky alone, were now possible.

Related post: SWR Meter - Easymatch
______________________________