[TowerTalk] Heliax and crankup towers

[Joe Subich, K4IK]

> Ever notice those mobile TV vans that do remotes from the field 
> and microwave the signal back to the station?  They get their 
> signals from their little microwave transmitters up their 
> telescoping mast to their directional antenna using a corrugated 
> feedline similar to Heliax and other similar low loss cables.  

Broadcast trucks do not use a Heliax type cable ... I designed 
and built those trucks for several years.  The RF cable is 
generally LMR-400 or equivalent.  We experimented with both 
Belden 9913 (solid center conductor, air dielectric) and 
Andrew Superflex (r) ... neither one held up as well as the 
LMR-400UF. 

Even at 2 GHz, the difference in loss between 3/8" Superflex and 
LMR-400 (about 1 dB) did not justify the added cost and weight 
of the more expensive cable.  

The older (analog) microwave radios generally had the final 
(power) amplifier at the top of the mast ... the new (digital) 
microwave units put an upconverter and power amplifier "top
side" and use a triaxial cable to feed power, modulated IF, 
and multiplexed control signals.  

> The feedline is coiled up around the mast but the cable is
> never really bent much.  In this kind of application, it is 
> quite durable. 

The majority of the masts are 50 to 55' tall (some as short as 
42' ... 25' in the SUV style ENG units) and although the bottom 
of the cable terminated at the roof line, the Nycoil that housed 
the RF and control cables was between 75 and 100 feet long (any 
shorter and it stretched too tightly putting excess stress on 
the mast).  The coil was about 36" in diameter when retracted 
and about 18" in diameter when extended.  When cold, the Nycoil 
could get quite stiff and brittle.  

> Doing the same to your telescoping tower might work out.

A 70' (crank up) tower would require a Nycoil at least 6 feet 
in diameter, and at least 150' long (I would recommend 200') ... 
I know of no source for Nycoils of that size.  It was difficult to 
"pull" a coil consisting of (1) LMR-400, two or three RG-59 
(for the camera on the top of the mast), and three "rotor cable" 
equivalent control cables in a 1.25" tube (which was the largest 
available at the time) ... I can't see anyone getting two .405" 
coax cables, rotor control and switch cables into a 1.25" coil.   

BTW, there are a few "Cell on Wheels" (a.k.a. COW) units that 
do use Superflex.  However, they are generally deployed in a 
single location for several days to weeks at a time and are not 
raising/lowering the antennas repeatedly.  


73, 

   ... Joe, K4IK