[VHFcontesting] Coax for VHF / UHF

Tue Aug 19 21:12:47 EDT 2008

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 at contesting.com
[mailto:vhfcontesting-bounces at contesting.com] On Behalf Of Rich Long
Sent: Tuesday, August 19, 2008 2:29 PM
To: vhfcontesting at 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