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Application Note # C27

CM5/52/9 -- Increasing Head Lead Length

The standard head lead supplied with CoverMasters is nominally 1·7m long, which is suitable for all normal operations (the CM5 had a straight lead, and the CM52 and CM9 have "curly" leads).

However, there have been some occasions when a user has wanted to operate a covermeter with a greater distance between the search head and electronics. Previous occasions of such a requirement have been various, but are typified by a desire to measure cover on a surface which is just beyond reach, without erecting any scaffolding, by tying the head to a wooden pole which will just reach.
In such circumstances, the standard head lead is not long enough, and needs to be extended.

CM5/52

Caution: CM5 vs CM52:

Note that the head lead for the newer CM52 is different to that for the older CM5, and the two lead types are not interchangeable.

Maximum length:

Tests at Protovale have established that the CM5 head lead length may be increased up to about 30m (100') without any loss of accuracy of measurement and with no need for special precautions.

The CM52 head lead includes additional conductors to allow either of two search heads to be connected (Standard or Midget), and to facilitate automatic recognition of which head has been connected.

It is not practicable to retain that facility in extra-long head leads or extension head leads; but providing the CM52 is to be used with one size of head only, a special extra-long coaxial lead can be used, up to the same 30m limit.

To meet these special applications, Protovale can supply, to special order, head leads of any length up to the maximum above, or alternatively extension leads to add on to an existing head lead.

"Do-It-Yourself" (CM5 only):

Users with adequate electronic workshop facilities can if they wish make up their own extension leads for a CM5 (only) providing they adhere to the following specifications:
Cable:
RF coaxial cable (to BS2316) of 50ohm characteristic impedance;
UR No. M76 strongly recommended as being more flexible than UR No. M43
( or, if a flame-retardant sheath is necessary, use LSF type RG58C/U ).
Plug connector:
BNC male plug, 50ohm impedance; any of the solder-, crimp- or twist-on types.
Socket connector:
either: BNC female cable jack, 50ohm impedance;
or:   another BNC male plug, plus a 50ohm socket-to-socket coupler.

CM9


The regular head lead supplied with the CM9 is a "curly" cable extending to about 1·7m length. Because of the larger number of conductors needed, it is not feasible to extend or replace this with simple coaxial cable.

Tests at Protovale have established that, using the straight version of the same multicore cable, the lead length may be increased to 5m (15') with no loss of accuracy, or up to 8m (25') if only the Standard (smaller) search head is used.

To meet these special applications, Protovale can supply, to special order, head leads of any length up to those limits above.

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