Fiberglass Base Station Antennas vs. Exposed Dipole Arrays

[railtrailbiker]

A two-way shop owner told me today that he now avoids using fiberglass base station antennas and prefers exposed dipole arrays instead.

He claims the fiberglass models all take on water after about 10 years, don't afford any lightning protection, and radiate less effective patterns than the exposed dipole models.

I have 3 repeaters in service, each using top of the line RFS fiberglass models. Should I consider replacing them?

Comments?

Thanks.



Tony

[Tom in D.C.]

The statement that one type of antenna is always better than another type at
all times is ridiculous on its face and has no value. If I were you I'd get my
advice from someone who approaches questions such as this with a lot
more of an open mind, and then, after some study, tells you what you need for a
particular situation.

[Jim202]

Part of the answer you may be looking for will depend on
just where the antenna will be located. Bear in mind that
the fiberglass stick type antennas normally have solder
joints between the different elements in the contruction of
the antenna. If the local conditions produce high winds, this
might now be the prime choice of an antenna. When I say
high winds, I look at mointain top sites with plenty of
elevation. The high winds will cause the antenna to flex
much more that a fixed dipole type antenna. This flexing
is the main cause for the solder joints to fail with time. Sure
you will get those on here to say that they have had these
sticks up for a number of years. Just don't buy a cheap
antenna for this application.

On the same hand, the fiberglass sticks seem to take a
lightning strike and can peal open like a banna. Those that
have seen it have that funny little chuckle. There are some
photos floating around the web sites of one of the results.
They also seem to have a lower tower loading performance.
If your in an area that has ice storms, the fiberglass is a
better choice. Most of the fiberglass stick antennas have
a metal top that is electrically grounded. This helps to keep
the lightning damage down. However it can be destroyed.

Again, the folded dipole antenna seems to stand up to the
wrath of the big spark much better. Being a DC grounded
antenna, they seem to survive the abuse a lightning strike
poses to them much better. Again there are those that will
say the harnes will be damged by a heavy strike. To that I
agree. Just don't forget if you have icing, the dipole will take
on more ice than a stick. The ice has a tendancy to detund
the dipole antenna. This can cause problems with the
transmitter with enough ice buidup.

Look at just how much sand may be blowing around. The
finish on a fiberglass stick antenna can be worn off by the
blowing sand. If it can cut through a telephone pole given
enough time, don't you think it will take the finish off of
fiberglass? Once the finish is gone, yes the fiberglass can
absorb moisture. If your in an area that has freezing, you
may see a faster decay of the stick.

There is no sure fired antenna to stand up to what mother
nature may dish out. The best you can do is make your
choice based on your local conditions.

Jim

A two-way shop owner told me today that he now avoids using fiberglass base station antennas and prefers exposed dipole arrays instead.

He claims the fiberglass models all take on water after about 10 years, don't afford any lightning protection, and radiate less effective patterns than the exposed dipole models.

I have 3 repeaters in service, each using top of the line RFS fiberglass models. Should I consider replacing them?

Comments?

Thanks.



Tony

[Bruce1807]

The unly real advantage of a quad stacked folded dipole or jpole is that you can tailor the pattern to suit. Whilst the surviveability at high wimds depends on whose antenna you buy, the actual windloading on the tower is critical if you are in the huricanne belt or on a mountain.
As Jim mentioned ice is a big factor on open antennas rather than the sticks.

As for the sand I find that hard to believe. All my sites are between 300 yards and three miles from the beach and I have no sand problems. Unless your antenna is less than forty freet above the ground. What you do need to take into account is the salt air. Fiberglass antennas deal better with this, however if in a hot enviroment the sun slowly destroys the fiberglass.

After 10 years sounds irrelevent well to me anyway.
I manage a PS system and replace all my antennas every three years regardless. A 10 year old antenna in my opinion is like a 10 year old radio and it time for a visit to the landfill.

Just my opinion but I buy TX/RX for all 800 applications
and Andrew/DB for VHF/UHF. My TX/RX stuff all survived a cat5 hurricanne although I am throwing them away at the moment before the next season begins but replacing with the same model and make.

Like Jim said though It depends where you live and what mother nature has to offer.

[psapengineer]

I too prefer the exposed dipoles. It's easier to control the pattern and when they break you get a high VSWR.

Fiberglass co-linear antennas shouldn't fill with water, they have a drain hole in the base near the connector. They do go intermittent if a joint between elements becomes unsoldered due to wind flexing. This sounds like scratchy Rx as the gain changes at the broken joint. When they do get sick they often don't exhibit any VSWR so they are hard to troubleshoot.

But, all that said, if it ain't broke, don't fix it.

I'd take signal level reading at each antenna from another distiant repeater and record it for future maintenance testing.

Bob

[Bruce1807]

If you have access to sitemaster you can do a hard print out.
I try to put all antennas on the sitemaster every six months while the site is being maintained and have found a VHF bad during wind gusts.

Sitemasters are worth the money if you can justify it.