Hi Rich.....this is a serious topic and unfortunately has a lot of "IFs,
ANDs, or BUTs".
The most important thing to focus on is LOSS. It is LOSS that we are
trying to avoid on VHF / UHF. The HF guys don't pay much attention to
this, because coax loss is MUCH lower at HF than it is at VHF+. Many an
HFer uses RG-213 coax, because it is "OK" at their low frequencies.
RG-213 is practically useless at VHF+ frequencies, because there is too
much loss(except for short jumpers). A secondary thing to focus on is
the LENGTH of the feedline(Of course, a longer feedline means more loss,
so you are back to worrying about loss). Another issue is this: How
serious are you about VHF / UHF operations??
I am attaching a coax / loss chart for your archives. This is a VERY
important chart. Spend some time learning about the various types of
coax available and what the losses are. Most guys only worry about loss
on TX. Their idea is that I generate 100W of RF power, I want all of
that to get to the antenna. Well, of course, that is not going to
happen. There will always be losses in the feedline. It is much more
important to realize that if you feedline has 1db of total loss on TX,
that it ALSO has 1db total loss on RX!! If you lose some TX power, you
can always just make a bit more. If you have a 1db loss, 21% of your
receive signal in the feedline is just gone forever. NOTE: I tried to
attach the Times Microwave chart, but the reflector refused my post
because it was too big. I have sent Rich the chart direct. Others will
have to find the original Times Microwave chart somewhere on the
Internet. Other such charts do exist on the Internet, but they are not
very complete. Do the best that you can.
Here is a small chart showing db loss vs power that gets to the top to
the antenna:
Relating db loss to(power-values rounded off)
db Loss Power In Power out %Loss
0.5db loss 100W 89W 11%
1db loss 100W 79W 21%
2db loss 100W 63W 37%
3db loss 100W 50W 50%
This shows that a 1db loss means that you will lose 21% of your TX AND
RX signal in the given feedline. Here are the most popular coax types
that are in use today:
1. LMR-400.....made by Times Microwave and (with different stock
numbers) by Andrew and CommScope. Unfortunately, everyone and their dog
have started making "cheaper" versions of this cable. Here, cheaper
means not as good. Stick to the good stuff made by reputable
manufacturers. This cable is the same size as RG-8 or RG-213, but it
has significantly less loss. The loss here is 1.5db / 100ft on 150Mhz.
There is also a version of this cable called LMR-400 UltraFlex. You
need this version for rotor loops or anywhere where repeated flexing
occurs. The "standard" LMR-400 is farly stiff and should not be used
for rotor loops or anywhere repeated flexing is anticipated.
2. 1/2" Heliax(LDF4-50A).....primarily made by Andrew, but also by
others. 1/2" Heliax is a good quality cable and the losses show that.
The loss of 1/2" Heliax at 150Mhz is 0.845db / 100ft.....almost half the
loss of LMR-400. In my opinion, 1/2" Heliax is the minimum acceptable
feedline--although I would try to keep the length of the feedline as
short as possible. ASIDE: Andrew makes a cable called 1/2" SuperFlex
Heliax. Its nomenclature is FSJ4-50B--its primary advantage is that
FSJ4 is much more flexible than LDF4. This cable is different from
LDF4-50A and has more loss. Actually FSJ4-50B is only slightly better
in loss than LMR-400 and costs a LOT more. Most of us only use FSJ4 for
short jumpers, taking advantage of its flexibility.
3. 7/8" Heliax(LDF5-50A)....primarily made by Andrew, but also by
others. 7/8" Heliax is really good cable. The loss of LDF5 at 150Mhz
is 0.458db / 100ft. This is one of the best feedlines available.
Thousands, no millions, of feet of this stuff have been put up by the
cell phone industry, so it has to be good stuff. Including the black
plastic jacket, LDF5 is just under 1" in diameter. This cable is a very
good choice even up into the lower microwave bands.
4. 1 5/8" Heliax(LDF7-50A).....primarily made by Andrew, but also by
others. 1 5/8" Heliax is about the best stuff commonly available to
VHFers. The loss of LDF7 at 150Mhz is 0.28db / 100ft. Except for
6M(where I use LDF5), I use LDF7 for all my feedlines in my VHF / UHF
contest station. LDF7 is really good stuff, but it is big and heavy.
Its outer diameter, with the black plastic jacket, is just under 2".
Again, a huge amount of this stuff has been put up by the cell phone
industry....they use LDF7 on the higher towers where losses are higher.
Here is another quickie chart of losses for various feedlines at 6M, 2M,
and 450Mhz:
Losses of common coax (per 100ft)
Coax type Loss-6M Loss-2M Loss-450
a. RG-58 3.1 6.2 10.6
b. RG-213 1.6 2.8 5.2
c. LMR-400 0.7 1.5 2.7
d. SuperFlex 0.73 1.29 2.32
e. LDF4 0.479 0.845 1.51
f. LDF5 0.257 0.458 0.834
g. LDF7 0.156 0.280 0.515
NOTE how high the losses are for RG-58 and RG-213......
So in summary, LMR-400 is acceptable feedline if the runs are short and
the frequency is fairly low. For better performance, you want to get
some Heliax, preferably LDF5 or better.
Heliax is commonly available on various surplus markets, ham flea
markets, and other such sources. Since the cell phone industry uses a
lot of this stuff, you might find a source by poking around any cell
phone gurus that you know. Bankrupt cell phone companies used to be the
primary source of surplus, but new, Heliax. Heliax does not degrade
like braided coax, because LDF types of Heliax do not suck water into
the core of the coax. Once you purchase Heliax, you do not have to buy
it again(baring physical damage such as cuting or crimping). Over the
long haul, Heliax is actually cheaper than braided coax.
The prices for Heliax vary all over the place. Unfortunately, we have
seen the price of Copper skyrocket lately. This makes Heliax worth more
to the scrap dealers and other such Destroyers. I would say that you
should be able to find LFD4 on the surplus market from $1 to $1.50 per
foot. LDF5 should be available around $2 to $2.50 per foot. LDF7
usually costs about $3 per foot. This may seem high to a new VHFer, but
there is just no substitute for good feedline. It also helps if you buy
in larger quantities. I have found that the cost per foot of a complete
spool of Heliax is MUCH lower than when you buy short pieces--especially
if the roll is surplus to begin with. DON'T buy new Heliax from a
Retail / Wholesale vendor. No one can afford it that way. I once
bought a 1280ft spool of 1 5/8" Heliax for $1100!!!!! Watch out for the
freight charges, especially in today's freight market. I paid $750 for
the shipping on that monster spool. Still, I got the entire spool for
less than $2 / ft. A really good deal.
One last thing, Heliax requires special connectors. You must have the
correct connector for whatever type of Heliax that you have. This is
one place that you cannot "cheat". Don't even try. Fortunately, I have
lots and lots of Heliax connectors, so if you need any, I have them.
I hope that I have covered this vast topic enough to answer your
question in a systematic way. If you have more questions, you may
contact me off the reflector.
Good luck in your VHF / UHF endeavors. 73's to all....Marshall K5QE
Wixson, Chuck wrote:
How much money you got.........?
-----Original Message-----
From: vhfcontesting-bounces@contesting.com
[mailto:vhfcontesting-bounces@contesting.com] On Behalf Of Rich Long
Sent: Tuesday, August 19, 2008 2:29 PM
To: vhfcontesting@contesting.com
Subject: [VHFcontesting] Coax
What would be the best coax to use for 2m SSB? I have RG-58 but I have
been told that it's lossy on 2M SSB.
Rich
KF6BKA
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