[600MRG] Telescopic Mast Vertical - Question

[Roger Graves]

Hi Andy,

This is an interesting situation. 

Re. using a telescopic mast as the vertical radiator.

My first thought is that it will be difficult to insulate the mast from the siding if it has HV on it as there can easily be several thousand volts after the loading coil and even if insulated from direct contact/arcing it could have a lot of capacitive coupling into the metal unit. Another potential problem is the resistance between all the mast sections. This is not usually an issue if the radiator is 50 Ohms, but if the radiation resistance is 1 Ohm or less, as it will be on 630, then it may be hard to keep the loss down.

Have you considered using the mast to support a wire vertical? I would still insulate the mast from the unit. For my aluminum mast I have an insulator at the top and run a wire spaced out from the mast from the bottom feedpoint to the top which then continues horizontally for the top loading wire. (Actually there is a pulley at the top with the halyard running down inside the mast so I can raise/lower the wire antenna.) 

With the wire antenna you could put the loading coil above the unit’s roof so the HV is both out of reach and above the unit’s roof to minimize the capacitive coupling from the near part of the coil and wire. The wire could continue down from the coil to near the ground where the actual feedpoint would be - where the radials connect to the matching system.

I hope I have made this idea clear. Just some thoughts that might give you some alternatives.
 
Re. the “how high” question - as high as possible :)

73, Roger

> On Aug 11, 2018, at 11:19 AM, Andy - KU4XR via 600MRG <600mrg at mailman.qth.net> wrote:
> 
> Hi all:
> 
> My recent antenna debacle is forcing me ( a good thing kinda ) to come up with a better and more permanent
> design.. After considering several types of setups, financial considerations, and one man maintenance has me
> settled on using a 40' Telescopic Mast as a Vertical radiator.. I have 2 questions :
> 
> #1: I will insulate the mast from ground.. How high above ground should I try to get the mast ? minimum & optimum
> 
> #2: I have no other place to mount the mast but to the side of my mobile home.. Lots of aluminum siding up
> 10 feet high, and , a metal roof .. I will use a stand off bracket to hold the mast at 10' AGL to the side of the
> mobile home.. it will be insulated ( mast wrapped with electrical tape ) - best I can do, and use dacron rope
> to guy the mast with porcelain insulators on the mast end .. Top Loading connected at the top section,
> and RF fed at the bottom of mast..
> 
> I realize the coupling, and interaction effects of being beside the metal body of the mobile home, so , will that
> make Question #1 a moot point ?
> 
> Thanks for any comments.. Hope to be back on the air soon:
> 
> 73:
> 
> Andy - KU4XR
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