Questions & Answers

 

1. What is the advantage of a two Tensioner installation?

 

Two Tensioner installations are highly recommended when the antenna is supported by two trees having lots of sway during wind gusts.  The benefit is that two Tensioners, a main and a reciprocal unit working together will compensate for two times the tree sway distance of a single unit.  Two Tensioners provide a larger safety factor and a more conservative and rugged installation.

wire antenna tensioner

wire antenna

2. What is a “ReciprocalTensioner

 

Both basic  Antenna Tensioner models ( Models 20-50 & 30-100 ) are available in a “Reciprocal” version designed specifically for use in a two Tensioner installation where units are installed at each end of the antenna.  Two Tensioner installations are highly recommended when the antenna is supported by two trees having lots of sway during wind gusts. A single Tensioner is recommended when the antenna has one fixed support and one tree support.

In a two tensioner system, antenna tension is adjusted by using the calibration markers on basic unit #1. Because both tensioners are connected together by the antenna, Unit #2 (reciprocal unit)  “reciprocates” and automatically extends its cable to the same tension. Two Tensioners will compensate for two times the tree sway displacement of a single unit and the total cable travel distance is doubled from 2.6’ to 5.2’.  Reciprocal units do not have the “soft stop” braking feature. Otherwise all tension characteristics are identical to the base models.

Instead of having multiple calibration markers, reciprocal units have just one calibration reference marker at the end of its movable cable. The antenna’s end insulator attaches to a stainless steel thimble. The reciprocal models have a “-R” part number suffix.

wire antenna tensioner unit

reciprocal wire antenna tensioner unit

 

3. How much tree movement will a Tensioner handle?

 

Model 20-50 will compensate for 2.6’ of to and fro sway @ 50 lbs tension and model 30-100 will compensate for 2.2’ sway @ 100 lbs tension. If the tree to and fro movement exceeds  this sway distance a “soft-stop” braking feature engages.  At this point the tensioner puts on the brakes and applies approximately 30 lbs more tension for each additional one inch of sway up to 350 lbs.

Two tensioners, a model 20-50 main and a model 20-50-R reciprocal unit at each end of the span, will compensate for 5.2’ of to and fro sway at 50 lbs.  A model 30-100 and 30-100-R will compensate for 4.4’ of sway at 100 lbs.  Two units working together will not double the tension.

Refer to the graphs below.

 

 

4. What is an optimal amount of wire sag on my antenna?

 

Sag depth of two to six percent (2-5%) of the wire span is a very practical and attainable design goal. Depending on the antenna span and weight distribution, experience shows that static tension in the range of 15-50 pounds will keep typical HF antennas nicely tensioned and will limit sag to about 5% or less of the span.  Vari-Ten variable Tensioners  can be adjusted to reliably deliver any necessary level of static tension within this 15-50 pound range.

To determine the amount of sag that your antenna will have go to Vari-Ten’s exclusive publication, DATA TABLES FOR WIRE SAG vs TENSION.  Nearly all antenna types are listed.

 

 

5. How does wire sag affect antenna performance?

Modeling of flat top antennas shows that the straighter and higher they are the better they will perform. But because of gravitational pull and the breaking strength of wire, tension and sag has practical limits and can never be eliminated, just controlled. So the question becomes, what is a practical and sustainable compromise for typical HF antennas?  Too much tension will break the wire or support rope and too little will result in excessive sag and less than optimal performance and appearance.

Unfortunately lots of amateur radio operators have not been able to install a reliable form of tensioning on their antenna.  So they protect it from excessive breaking forces when the support trees sway by leaving a large amount of slack in the antenna wire…enough to compensate for the distance that the trees sway.  However this causes excessive sag and degraded performance.  Vari-Ten Tensioners solve this problem.

Lower height means lower performance

Loss of height translates to loss of performance and this is a compelling reason to properly tension any wire antenna. Here is a typical example of how much antenna height will be sacrificed by adding slack to compensate for tree movement.

  • Let’s assume that you have a 136’ dipole antenna tensioned to 25 lbs. It has a feed point insulator and 50’ of 6” ladder line having a combined center weight of 3 lbs. The ends of the antenna are supported by trees at 50’. At 25 lbs tension the feed point (area of highest radiation) will sag 6.9’ and will be 43.1’ above ground.  Take the same antenna and add just 3’ of halyard slack to compensate for tree movement and you will add another 8.3’ of sag. The antenna’s area of highest radiation will fall to just 34.8’ above the ground and 30% lower than the ends.

 

 

wire antenna

 

 

6. What factors affect wire sag?

 

In amateur radio work, it is generally accepted that antenna wire can be worked to about 30% of its rated breaking strength.  Given the limitations of wire strength, antenna sag then becomes a function of the length of the span, total antenna weight, how the weight is distributed across the span, and the applied tension. A single wire tensioned between two fixed supports will form a catenary curve with the deepest sag at the center of the catenary.   Center loading with the concentrated weight of a feed line will distort the curve into a more pronounced “vee”.  Sag is reduced by increasing the tension on the wire.

7. How much sag will my antenna have?

 

Refer to Vari-Ten’s exclusive publication titled DATA TABLES FOR WIRE SAG vs TENSION.

These tables list just about every kind of antenna, 160-10m, and the amount of sag it will have when tension is applied by a Vari-Ten Tensioner.  The data tables were compiled from sag tests on full size antennas erected on a test range. Link to Data Table

 

8. Can Tensioners be used on any kind of wire antenna?

 

All types of popular 160-10m wire antennas will be protected when they are supported by a Tensioner . Single span antennas such as Dipoles, Doublets, Windom, G5RV, Carolina Windom, OCF, Inverted L’s, long wires, etc can have just one end or both ends supported by a Tensioner.

Double span antennas such as Inverted Vee’s and OCF’s or similar antennas supported at the center insulator (two separate spans supported at the center) can use a main or a reciprocal Tensioner at each end of the Vee elements. If a tree top pulley is used, Tensioners can also be mounted vertically at ground level with the open end anchored to a ground stake or eye bolt at the base of the tree.

Loop antennas…Single or multiple Tensioners can be used with a horizontal delta loop or quad loop depending on the installation circumstances and support trees.  Loops are often tensioned by running the corner halyards through a top pulley and down to a vertically mounted Tensioner at ground level.

wire antenna tensioner systems

 

 

 

9. What about use with “stealth” antenna installations?

 

Tensioners are ideal for “stealth” installations because they are designed to be concealed high in the tree top canopy. The barrel of the Tensioner is double coated with a durable outdoor olive drab camouflage paint that blends in with the branches and foliage. Tensioners also eliminate the need for a counter weight or other tensioning device at ground level that can be unsightly, a dangerous attraction for children, or a problem with CC&R’s. Also refer to wire breaking strength  Q & A #11.

 

wire antenna tensioner pictures

 

 

10. Can Reciprocal Tensioners be used as stand alone units?

 

Yes, Reciprocal models have the same tension characteristics as the basic models but do not have the full range of calibration markers on the movable cable.  There is just one reference marker and a cable thimble at the end of the cable.  Reciprocal units are ideal for installations where the Tensioner is installed at or near ground level and the tension adjustments can be estimated without the aid of multiple calibration markers. Inverted Vee’s and ground anchored vertical installations are ideal stand alone applications for the reciprocal models.

 

 

 

11. Why is wire breaking strength important?

Your wire antenna is a system of components linked together that must be tensioned to keep it in position.   All components, including the antenna wire, need to have sufficient strength to withstand the range of day to day tension during calm days as well as the peak  tension that it will see when gusty winds move the support trees.  During periods of strong gusts (which typically last 2-3 seconds), it is not unusual for Tensioners to fully extend and  momentarily apply tension that is at or above its nominal range.  The antenna wire and all of the other components such as supports, rope, insulators, balun, etc., need to routinely withstand this momentary peak stress.  If there is a component failure the antenna will obviously fall and the Tensioner calibration markers could be damaged if the extended cable instantly retracts back to its normal position.

As a rule of thumb, you need to use antenna wire that has a breaking strength rating of 300 lbs. or higher.  Halyard breaking strength should be 700 lbs. or higher.  These values are well within the published breaking strength ratings of the most commonly used and recommended antenna materials.  Light duty commercially available “stealth” antennas and many types of “stealth” antenna wire have breaking strengths far below 300 lbs. and are not suitable for use with Vari-Ten Tensioners.

Go to Table 2 to view antenna wire breaking strengths, or consult the ARRL Handbook Wire Tables, or contact your wire supplier.

 

Table 2

Wire breaking strength in pounds

Wire wt/ft

 

Feed line wt/ft

AWG

40%Cu clad steel

Hard drawn copper

Soft drawn

copper

Cu Flex Weave

7 strand

clad Cu

 

 

 

 

   RG-6

 

 

 .030

12

917

337

200

170

---

,0198

 

   RG-8    RG-213

.115

14

550

234

125

128

349

.0124

 

RG-8X

600 ohm

.040

16

381

135

70

n/a

---

.0078

 

RG-11

.092

  18

280

85

50

n/a

---

.0049

 

450 ohm

.026

 

RG-58

.029

 

RG-59

.032

12.  Do I need a tree top pulley? 

A tree top halyard pulley is needed if you are installing a Tensioner vertically at ground level.  If the Tensioner is installed in the tree top a halyard pulley is not needed since the halyard stays stationary after the antenna is erected and tension is applied.  However a  pulley can be a convenience if you frequently raise and lower your antenna for adjustments.

13.  Why not use bungie cords or rubber cargo straps for antenna tensioning?

Frequent reports from experienced ham radio operators show that antennas that have been tensioned with bungie cord or cargo straps eventually end up on the ground after high winds or icing. During the process of developing Vari-Ten Tensioners, research and tension testing was done on many sizes of marine and military grade bungie cord and on all standard lengths of natural and EPDM rubber cargo straps ranging from 11” to 44”. Both of these materials were found to be poorly suited for antenna tensioning. Bungie cord and rubber cargo straps have a fairly short life in a 24/7 outdoor environment and tension tests showed that neither material had an adequate range of elasticity or the necessary tension to do a good job of handling tree sway and antenna wire sag.  After they are stretched to the needed tension (working tension) there is not enough reserve stretch remaining (dynamic tension) to handle typical tree sway in high winds.  Both materials eventually lose elasticity and take a permanent set under constant long term tension. There is also a significant decrease in breaking strength with age and weather exposure.