Going discreet with a Random Wire Antenna and 9:1 unun.

(When winding the toroid wear safety spectacles as the wire could easily flick into your eyes.)

You live in a place where there are neighbours. You have space available to place some kit, but not that much. Even if you decided to use that space, you wouldn't want your neighbours to imagine what all that stuff is about. If your neighbours found out chances are that one day you would end up having a discussion with one of them, not because you disturb TVs and they hear you talking in their toaster, but because they don't like what they don't know.

If all of the above sounds somewhat familiar, welcome to the SOC - the Stealth Operator's Club. We are people who, for one reason or another, have to go discreet - or even stealthy.

An ideal choice is the magnetic LW unun. With this simple piece of equipment you can use 20m of antenna wire across all HF bands using the inbuilt ATU of your TX/RX.

LW TX RX
LW TX RX
My effort for an end fed long wire antenna using alternative components. End fed LW antenna most convenient but often requires its high impedance, typically 450 Ohm, to be matched to common 50 Ohm TX/RX. Maintaining the trifilar winding spacing helps to minimise common mode currents IMHO. With the use of an antenna tuning unit this project works well on all HF 160 to 10 metres TX amateur radio bands. Also likely useful on its own for SWL broadcast bands RX. If you require to connect an earth or counterpoise use the chocolate block where the RG58C/U terminates or a coupler on the coax PL259 with shield/earth terminal. YMMV. As shown not suitable for long term external use.

Caution: It is possible to exceed the power rating of a core and the performance of the UNUN may be degraded during high SWR causing heating of the core. If the core is over heated its magnetic properties will most likely change and your UNUN will not work efficiently and may even damage your radio or ATU. The highest power I've used so far is 15 Watts.

I use a FT240-31 toroid which is generally regarded useful for HF, cost about 10.

MKll version below using enamelled copper wire 8SWG 1.250mm Maplin YN81:
MKll version.
MK3 version below similar to MKll but using two toroids stacked and held in place with PTFE tape. Using two toroids may increase the power handling. This is the version I continue to use:
MK3

You may find Common Mode Currents can be resolved with my Choke Balun

Choke Balun re Common Mode Currents.


Source: Phil VK5SRP, John M0UKD & M0ZZM.

How long is a long wire? As I recall it used to be 132 feet but that was many moons ago possibly before the latest WARC bands I muse.

Quote: "The secret to using an end-fed wire is to pick a length that will present a reasonable impedance on the bands of interest so that it is possible to match the wire with a tuner. The key to this is to avoid lengths that are multiples of a half wavelength on the amateur bands, as this will present a very high impedance that will be difficult for many tuners to match. Fortunately someone has already figured out what this magical length is. Alan Chester G3CCB (SK) wrote an excellent article called 'Taming the End-Fed Wire' ('The Antenna File' 12.74, RGSB, page 118) he looked at this issue by plotting various wave lengths and proposed 26.5m (86.9 ft) for 160 - 10M use."

Source: Michael Babineau, VE3WMB

I use a 25m LW with 9:1 unun, choke balun, tuner and optional 10m length counterpoise around skirting board of the shack. It tunes up fine on all HF bands. However, I would urge caution on the power used. I suggest 40 Watts maximum. I have no technical data on this it's just an opinion. 73 Bob Houlston G4PVB

p.s. myantennas.com suggest following lengths for EFLW End Fed Long Wire antenna:

44'   13.4m
53'   16.2m
73'   22.3m
88'   26.8m
117' 35.7m

Could operate without counterpoise if coaxial cable at least 25' / 8m but ground connection recommended.

BTW: Is a Choke Balun needed?
Generally speaking, no. Depending on the quality of your counterpoise, current splits between the counterpoise and coax shield.

You can use a choke balun, but it must be used at the rig end of the coax feed, NOT next to the UNUN or you will have no counterpoise and the 'unit' will not work.

100pF compensation capacitor
An optional 100pF capacitor is part of the UNUN antenna matching circuit. It is placed across the primary of the UNUN to improve high frequency UNUN performance.

Helps to compensate mainly for UNUN primary leakage inductance.

The value of 100pF works well for most UNUNs, small or large, low power or high power.

Typically uses a ceramic disk "blue cap" 3KV or more. These caps typically have a high current rating as they must handle several amps of RF current.

Voltage rating can be increased by putting two 200 pF caps in series. Current rating can be increased by putting two 47pF caps in parallel.

For QRP rigs, a silvered mica 100pF 500V or 1KV rated cap is fine. Search EBAY for 100pF silver mica, mouser.com, digikey.com

Sources of 100pF high voltage caps:
EBAY (search for 100pF capacitor. Select a "blue cap", 3KV or more
Mouser P/N 81-DHR4E4C221K2BB (220pF, put two in series. Ceramic disk 15KV rated, $1.23 each

Source: http://gnarc.org/wp-content/uploads/The-End-Fed-Half-Wave-Antenna.pdf

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