Hello! I'm performing maintenance on several HF whips. The directions
call for megging the HF antennas to make sure the resistance to ground
is greater than 200 M Ohms. How critical is this measurement.
Sometimes, the Meg reading has gone down to 86 K ohms, before being
replaced, but, the boat still has good comms.

Also how important is it to have a good ground on these antennas. I
do not know a lot about antenna theory, but it would seem to me that
a good ground would be important for small UHF antenna using a ground
plane, not a 35 ft whip. However my boss says that by ensuring the
antenna base has a good ground, the megger reading will somehow go up.
This I do not understand.

How someone can answer my questions, and tell me which of us is right

Thanks
-Regards

No reason to believe that the resistance to ground has to be
as high as 200 Megohms in order for the antenna to work,
although new antennas might reasonably be expected to show
that level of resistance.

If you are measuring leakage resistance to ground of 86
kilohms, that is most likely due to a dirty, salt-encrusted,
or cracked base insulator. It should probably be replaced if
it can't be cleaned or repaired. Whether 86 kilohms is
enough to interfere with proper operation of the antenna
again depends on frequency of operation, given a 35 foot
physical length. When the insulator is wet, the resistance
could be even lower.

The quality of ground is important for communications, to be
sure. However, with a 35 foot whip, the importance will be a
function of frequency. When the whip is near a
half-wavelength, for example, ground quality becomes far
less important to antenna efficiency.

I am aware of no mechanism by which quality of ground would
affect the dc leakage resistance of a whip antenna base.

Good luck.

Chuck

Johny Looser wrote:
Hello! I'm performing maintenance on several HF whips. The directions
call for megging the HF antennas to make sure the resistance to ground
is greater than 200 M Ohms. How critical is this measurement.
Sometimes, the Meg reading has gone down to 86 K ohms, before being
replaced, but, the boat still has good comms.

Also how important is it to have a good ground on these antennas. I
do not know a lot about antenna theory, but it would seem to me that
a good ground would be important for small UHF antenna using a ground
plane, not a 35 ft whip. However my boss says that by ensuring the
antenna base has a good ground, the megger reading will somehow go up.
This I do not understand.

How someone can answer my questions, and tell me which of us is right

Thanks
-Regards

Ok! Thanks for the info.

-Regards



On Tue, 15 Mar 2005 10:13:40 -0500, chuck wrote:

No reason to believe that the resistance to ground has to be
as high as 200 Megohms in order for the antenna to work,
although new antennas might reasonably be expected to show
that level of resistance.

If you are measuring leakage resistance to ground of 86
kilohms, that is most likely due to a dirty, salt-encrusted,
or cracked base insulator. It should probably be replaced if
it can't be cleaned or repaired. Whether 86 kilohms is
enough to interfere with proper operation of the antenna
again depends on frequency of operation, given a 35 foot
physical length. When the insulator is wet, the resistance
could be even lower.

The quality of ground is important for communications, to be
sure. However, with a 35 foot whip, the importance will be a
function of frequency. When the whip is near a
half-wavelength, for example, ground quality becomes far
less important to antenna efficiency.

I am aware of no mechanism by which quality of ground would
affect the dc leakage resistance of a whip antenna base.

Good luck.

Chuck

Johny Looser wrote:
Hello! I'm performing maintenance on several HF whips. The directions
call for megging the HF antennas to make sure the resistance to ground
is greater than 200 M Ohms. How critical is this measurement.
Sometimes, the Meg reading has gone down to 86 K ohms, before being
replaced, but, the boat still has good comms.

Also how important is it to have a good ground on these antennas. I
do not know a lot about antenna theory, but it would seem to me that
a good ground would be important for small UHF antenna using a ground
plane, not a 35 ft whip. However my boss says that by ensuring the
antenna base has a good ground, the megger reading will somehow go up.
This I do not understand.

How someone can answer my questions, and tell me which of us is right

Thanks
-Regards