Antennas

New PageWeatherproofing the Feedpoint — Step by Step

  1. Clean the connections first. Use a small wire brush or fine sandpaper on the copper wire and hardware to get any oxidation off before you seal anything in.
  2. Apply a thin coat of dielectric grease (like the kind used on spark plugs — cheap at any auto parts store) over the bare metal connections and screw hardware. This displaces moisture and slows future oxidation.
  3. Wrap the connection with regular electrical tape first — just one snug layer to give the self-amalgamating tape something to grip and to hold everything mechanically in place.
  4. Wrap over that with self-amalgamating (self-fusing) tape. Start a couple inches below the connection on the coax, wrap up and over the whole connection, and come back down a couple inches past it on the other side. Stretch it as you wrap — that's what makes it fuse to itself and seal out water. No adhesive needed, it bonds to itself.
  5. Optional but good — one final layer of regular electrical tape over the self-amalgamating tape. Self-amalgamating tape can get sticky and collect dirt over time in UV exposure. The outer electrical tape layer protects it.
  6. Check the coax entry point into the house wherever it comes through the wall or window. That hole needs to be sealed too — a weatherproof grommet or even just silicone caulk around the cable where it enters. Water loves to wick down a coax cable and into your wall.

Dual Band Dipole Antenna — 2M / 70cm Build Guide

Overview

This guide covers building a dual-band dipole antenna for 2 meters (144 MHz) and 70cm (440 MHz) using thick copper wire, mounted on a PVC mast and fed with KMR-240 50-ohm coax.

Materials

How It Works

Each leg of the antenna is one continuous wire with two straight radiating sections:

The feedpoint sits at the junction between the two straight sections — where the 19.5 inch and 6.5 inch sections meet on each leg.

Wire Measurements

Section Length per leg
2M radiating section 19.5 inches
70cm radiating section 6.5 inches
U fold (remainder) Whatever is left — shape does not matter

Cut each leg with a few extra inches to allow for the U fold and for trimming at the feedpoint end.

Step 1 — Shape Each Leg

Working from the feedpoint end outward on each leg:

  1. Measure and mark 19.5 inches — this is your 2M section
  2. Continue another 6.5 inches past that mark — this is your 70cm section
  3. Fold any remaining wire back into a U shape — keep it tidy and not touching the other leg or the straight sections

Both legs should mirror each other. One points up from the feedpoint, one points down.

Step 2 — Mount Legs to PVC

Attach each leg to the PVC mast using screws and washers. The legs run parallel to the mast with the feedpoint junction at the center. The screw/washer hardware holds the wire mechanically — the electrical connection will be handled by solder in the next step.

Step 3 — Prepare the Coax

At the antenna end of the KMR-240, strip back enough jacket to cleanly separate:

Tin both the coax conductors and the antenna wire ends with solder before joining them. Solder flows onto pre-tinned surfaces much more cleanly.

Step 4 — Solder the Feedpoint

The two solder joints sit at the junction between the 19.5 inch and 6.5 inch sections on each leg — right where the two straight sections meet.

Step 5 — Make a Choke Balun

Right at the feedpoint, coil 8 turns of the KMR-240 into a roughly 6-inch diameter loop. Zip-tie the coil to hold its shape and secure it to the mast.

This stops RF current from running back down the outside of the coax shield, which would otherwise cause the feedline to act as part of the antenna and degrade performance.

Step 6 — Weatherproof the Feedpoint

Since the antenna is taken down periodically for inspection, tape is not used here — dielectric grease alone is sufficient for the Buena Park climate and makes the connection easy to service.

  1. Inspect the solder joints visually — no cracks, no shorts
  2. Apply a generous coat of dielectric grease over every solder joint and exposed bare metal at the feedpoint
  3. Ensure all bare surfaces are fully coated — the grease displaces moisture and slows oxidation
  4. Reapply fresh grease each time the antenna is reinstalled

Check and reapply at least once a year, or after any extended period of wet weather.

Step 7 — Run the Feedline

Run the KMR-240 from the feedpoint to the radio. Notes for the cable run:

Connector Reference

Device Connector
Baofeng UV-5R SMA-Female (on radio)
KMR-240 cable ends N-Male
Most RTL-SDR dongles SMA-Female
Some cheap SDR dongles MCX
Old TV coax F-connector
Typical mobile radio PL-259 (UHF)

Use quality silver-plated adapters — avoid no-name adapters especially for 70cm where connector losses add up.

Notes