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I don't see a choke at the feed-point Alexis ?

Quick test to try,
Disconnect the ground strap & see if that changes vswr with the mast at the different heights you have tested in the chart above,

If it does you have common mode on the mast & ground strap which will change with its electrical length & been grounded or not grounded at the ground rod,

Also as you raise it up you have less coax laying on the roof,
if you have common mode on the coax that could also change the tuning.

If its not that it could be that your roof has rebar in it & that's effecting your tuning.

Distributed capacitance to ground or other conductor under the antenna will effect tuning,
more height less distributed capacitance.

[QUOTE="bob85, post: 746314, member: 18"] I don't see a choke at the feed-point Alexis ? Quick test to try, Disconnect the ground strap & see if that changes vswr with the mast at the different heights you have tested in the chart above, If it does you have common mode on the mast & ground strap which will change with its electrical length & been grounded or not grounded at the ground rod, Also as you raise it up you have less coax laying on the roof, if you have common mode on the coax that could also change the tuning. If its not that it could be that your roof has rebar in it & that's effecting your tuning. Distributed capacitance to ground or other conductor under the antenna will effect tuning, more height less distributed capacitance. [/QUOTE]

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