In recent months 7 MHz NVIS propagation has taken a fall off the propagation-cliff to be replaced by its 5 MHz and 3.5 MHz siblings.
As the sun’s natural 11-year solar cycle moves closer to a solar minimum, radio amateurs are experiencing a change in the MUF. Since January this year the NVIS frequency has moved south from a comfortable position above 7 MHz to 5 MHz and is likely to move lower in the next 12 to 24 months. Unfortunately radio amateurs in Australia don’t have an allocation in the 60m band (5 MHz) therefore the next lowest frequency to attempt NVIS propagation is 3.5 MHz. Conveniently 3.5 MHz is available to all licence classes including the Foundation class.
For SOTA activators operating in the south-east of Australia and who get of bed early to be on a summit by 2200 to 2300 UTC (9 am local), now is a good time to get ahead of the crowd and operate in the 80m (3.5 MHz) SSB or CW segments. If you operate in VK check Ian VK1DI’s blog on local daytime 80m propagation.
What do I reqire to operate portable on 3.580 MHz SSB or lower in the CW sub-band? Well for most radio amateurs operating portable on 80m means constructing a new 80m 1/2 wave Inverted V dipole, almost twice as long as a 40m 1/2 wave dipole. I have a lightweight 40m Inverted V dipole designed for operation in the lower end of the 40m band centered on 7.090 MHz, how do I upgrade my portable SOTA 40m dipole to operate on 3.5 MHz?
Homebrew is the answer! Don’t layout hundreds of $$$ dollars on a commercial solution, go ahead, make your own..
I have a 40m Inverted V dipole for SOTA, lets add two clip-on wire extensions, one wire extension to each end of the existing dipole wire elements. You now have a working 80m 1/2 wave dipole while maintaining the ability to operate on 40m. 🙂
- 4 nylon Sister Clips (Road Tech Marine, Canberra)
- 12 small cable ties (Bunnings Hardware)
- 2 lengths of lightweight wire 9.8 metres long (80m extensions for 3.580 MHz SSB). I used a product similar to Dx-wire.
- 2 pairs of auto bullet connectors
- 2 * 40 mm lengths of 6 mm (1/4 inch) fiberglass rod or other suitable insulator material (from the junk box)
- 2 * 6 metre lengths of 2 mm cord (Rope Galore, Cooma)
- Wire winder
- Ratchet Crimping Tool
- Wire Stripper or small Side Cutter
- Tape measure
Before you cut the wire extensions, first decide on the type of end insulator you will use at the far end of each 80m extension. You must allow for a small fold-back length to secure the insulator. In my calculations I have allowed 100 mm of wire for a fold-back, you can adjust the length based on your insulator design. Note, after adding the end insulator, the physical length of each extension is 9.7 metres. Measure and cut two lengths of wire 9.8 metres long, measure twice cut once. Next using a multi-meter check the continuity of the wire, you can’t see hidden breaks.
Photos: © Copyright 2016 Andrew VK1AD
Now follow the instructions and use the pictures as a guide, no soldering required!
If you haven’t seen a Sister Clip before here it is.
- Unpack or uncoil your existing 40m 1/2 wave dipole giving you access to the antenna end insulators
- Add one Sister Clip to each end of your 40m 1/2 wave dipole. Untie your support cord/string from the end insulator, feed the cord through the Sister Clip and retie the cord around the end insulator. Ensure the Sister Clip is held secure against the insulator.
- Next, and assuming your 40m dipole end includes a small length of wire folded back, unfold the wire to make one half of a link, about 100 mm (4 inches) long. Apply two small cable ties to prevent the insulator from sliding along the existing antenna element.
- Add a female bullet connector to the short fold-back section to form one side of the link (left).
- Take one length of 9.8 metre wire and feed one end through a Sister Clip. Strip the wire outer insulator (12 mm) to expose the copper wire.
- Crimp the male bullet connector to the end of the 9.8 metre wire.
- Feed the Sister Clip back along the extension wire, about 100 mm (4 inches) and apply two small cable ties to hold the Sister Clip in place.
- Next engage both Sister Clips and the two halves of the bullet connector. The link formed by two bullet connectors is now included in the physical length of the wire element to form one half of the 80m dipole. The link should be ‘floppy’ and not under any stress. The Sister Clips should be carrying the load on the joint not the bullet connectors.
- With a multi-meter check for a low resistance across the link. If the value is not close to zero ohms, check the bullet connectors are properly engaged and the crimp is tight and secure.
- At the far end of the extension wire add an end insulator of your choice (see picture)
- Secure the insulator in place with one or two small cable ties
- Add one length of 6 metre cord to the end insulator. I managed to raise the antenna ends 1 metre above the ground
- Repeat steps 1 to 12 for the other side of the dipole. Don’t mix up the bullet connectors.
- With both extensions finished next wind the extensions around a winder, ready for deployment.
If for some reason a pair of Sister Clips disengage (unlikely) the round bullet connectors should disengage under the tension and allow the antenna extension to separate from the 40m dipole without damage.
Operating on 80m
In the photo (Castle Hill SOTA activation 7 August 2016) the 80m link is engaged and operating on 3.585 MHz. For operation on 40m slowly pull the cord to lower the antenna and disengage the link bullet connectors, leaving the Sister Clips engaged. Allow the antenna to return to its elevate position. This antenna is supported by a 10 metre fiberglass telescopic pole, commonly known in Australia as a Squid Pole. My pole has sufficient flexibility which allows me to pull the link down to chest height.
80m extension end insulator with a small length of wire folded back. I have allowed for a 100 mm fold back. When you measure the wire extension’s physical length (9.7 metres) measure from the male bullet connector to the end insulator, adjust as required.
Two 80m extensions (9.7 metres) wound on a TET-Emtron winder
Have loads of fun operating on 80m with a simple 9.7 metre clip-on extension to each end of a 40m Inverted V dipole. 🙂
If you have questions please contact me via the ‘contact me’ page. Remember my 80m extension dimensions are for operation on 3.585 MHz using a 40m Inverted V dipole centered on 7.090 MHz. The antenna feed point apex is about 9 metres above the ground.
Reference / Links
Published: 23 August 2016
Last Update: 25 August 2016