826 wrote:
Hi,
Here is some more info. The depth of the coax is 6". Its length is 75 foot
of RG-213. The ground here is below sea level (3 ft). It was reclaimed from
salt water by wind driven pumps in the 14th and 15th century. The
conductivity of the ground is great for antennas. But not sure about buried
feed lines.
...

I, like Richard C., would like to encourage you to the use of a 1:1
current-choke/balun, either of a toroid core of proper material--or even
a ferrite rod, beads, etc. -- and installed at the antenna feed point or
both ends of the coax (xmitter also) ...

I would have to install software and check out a couple of things. But,
I suspect, and especially at ~28+ mhz, that capacitive load on the outer
conductor can't help look like anything other than/near a direct short.
Even the rf down the inside of the braid/shield must be tempted to
that path if it nears or is less than ~50 ohms (well, some
possible/noticeable effect/affect.)

The coax I have buried, I always encased in ~1 inch PVC--possibly an
overkill ... but hey, it was cheap when I bought it--I ran the UNUN
(unbalanced-to-unbalanced current balun), just don't remember if I had
to, or not ...

Just my two cents worth--feel free to disregard if someone ventures
better/more-accurate data or proofs ... but then, you already knew that.
;-)

Regards,
JS