Antenna wire

Started by rgp415, March 23, 2019, 01:57:18 pm

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rgp415

March 23, 2019, 01:57:18 pm Last Edit: March 24, 2019, 02:15:44 am by rgp415
Is it ok to connect shorter runs of wire to make one long antenna run? Any cons to this? Thanks to everyone for your input.

gil

Absolutely, no problem.  :)

Gil.

Jim Boswell

When joining wire for a HF antenna, use strong, soldered connections. For portable antennas I use 18ga. wire, for long term antennas I use 14ga wire. de KA5SIW

Alpha_Greywolf

Quote from: rgp415 on March 23, 2019, 01:57:18 pm
Is it ok to connect shorter runs of wire to make one long antenna run? Any cons to this? Thanks to everyone for your input.


A linked dipole is just lengths of wire temporarily attached to each other to increase its length.
Mine is cut for 20/40 with crimped and soldered spade connectors to bridge the gaps.

It has been outside in high winds and all weather, the only time it broke was when the suspension pole was blown over and the wire snagged. A quick replacement of the spade connector and it was good to go again.

RFExplorer

Speaking of antenna wire .... I read that it's common practice to cut wire longer than the formula length and fold it over on itself. Trimming is done by changing the fold point to adjust the wire length. This method certainly avoids the situations where I exclaim, " I can't understand it, I cut it twice and it's still too short !"

But does this method work when you are using insulated wire ?

Ed

gil

QuoteBut does this method work when you are using insulated wire ?

It should... I prefer to spool the wire on a stick... What I usually do though is simply cut the wire little by little, noting how many KHz change I get per inch.

Gil.

bobtennis

Winding on a stick or folding wire back on itself will get you ballpark measurements, but may require a lot of extra trial and error testing after cutting the wire. These methods can induce capacitance or inductive loading effects which may skew measurements.

One alternate method I used, and it worked really well, is to take a reading of the lowest SWR point at your initial wire length and note that frequency. You can use the standard 468/frequency to determine an initial length.

Multiply the initial length times the lowest measured SWR frequency (i.e., take test measurements) to find an adjustment factor, then calculate adjusted length using this factor for best resonance (SWR) at your desired frequency. You can use this method whether you have an antenna analyzer or multiple trial measurements, as long as you can measure lowest SWR point (but the analyzer makes this really easy!)

Restated:
original length x original frequency low SWR point  = adjustment factor; then divide the desired frequency into this factor to get the corrected length.

For a hypothetical example:
Given - Initial trial length of 34 ft per dipole antenna leg, and desired 40m frequency is 7.100 mhz.

If measured low SWR for initial length of 34 ft antenna leg is at a frequency of 7.00 mhz, to find the length needed 7.100 mhz, set up the equation:

34 ft x 7.0 mhz = 238 (new adjustment factor) :  new adjustment factor 238 / 7.100 mhz = 33.5 ft new length (or 33 ft, 6 in - remember to convert the decimal to inches!).

This should get you very close to the correct length, and, it also takes into account your particular circumstances which cause a deviation from using the standard formula 468/freq mhz (or 234/freq for each dipole leg), such as surroundings, height, and other factor affecting antenna resonance.

Metric system users will need to make appropriate adjustments in lengths and calculation formulas.

Although this example used a dipole for illustration, there is no reason not to use it for other type antenna adjustments. For example, I used it to adjust a single leg sloper coming off a rooftop tower counterpoise successfully. It really just boils down to using a math tool to calculate a proportionate change.