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[W5OT]

I've been running a Diamond F23H with my Icom 746Pro and a 50 ft piece of Rg-58 with great results.  Full power out across the entire 2 meter band.  I decided to upgrade the coax to LMR-400.  The 50 ft of RG-58 just barely reached, so I have a tower grounding clamp that I ran the LMR-400 to and another 12 ft piece of LMR-400 to the radio.  Now, my bandwidth is narrowed to about 2MHz with the power folding back at the low and high ends of the band.  The antenna is NOT mounted on the tower with the coax grounding clamp.  I'm thinking this might be the issue.
I'm wondering if bypassing the grounding clamp at the base of the tower will help.  Or, do like I did with the RG-58 and run directly from the antenna to the radio with 50 ft and eliminate the extra 12 ft. 
What say ye all??

[WB6BYU]

The RG-58 has higher loss, which means the SWR will be
lower at the rig.

Whether or not that explains the whole problems is a good
question, but it certainly will have an impact on the observed
bandwidth of an antenna.

For example, with a 50' length of RG-58, an SWR of 3 : 1 at
the antenna will look like 1.6 : 1 at the radio due to the cable
loss.  Using 50' of LMR-400, it will be 2.4 : 1 instead.  If your
radio only handles up to 2 : 1 SWR, that may make a difference.

[W3PX]

The 50 feet of lossy RG-58 masked the VSWR of the antenna and provided a better match to the rig with less reflected power.

With the new, better feedline now is the time to look at the antenna match and see if it needs to be tuned a bit better.

[AJ8MH]

It could be that the LMR-400 is showing more reflected because it has less loss.  Could be.  What does the documentation say about antenna bandwidth?

It appears that the F23H is mounted in the clear on a separate tower or pole from the coax grounding clamp on the tower.  50 feet of LMR-400 runs from the antenna over to the tower with the grounding clamp.  Here, there is a connection to 12 additional feet of LMR-400 that runs to the shack.

I doubt it's where the grounding clamp is mounted.  It could be the grounding clamp itself.  Try by-passing it.  Put a dummy-load on the end of each coax and see if you notice any abnormal readings.  Also, are you sure your connectors are OK?   

[AI5BC]

I've been running a Diamond F23H with my Icom 746Pro and a 50 ft piece of Rg-58 with great results.

No, you just thought it worked great. Where you went wrong is you do not understand coax loss and how it interacts with the antenna readings. The RG-58 excessive losses at that distance/frequency masked your antenna problem. With a 100-foot section of RG-58 and nothing connected to it, either an open or shorted out, can make you think it is the best antenna you can have. Works great with a flat SWR = 1.6 across the entire 2-meter band. At 70 cm has a flat SWR = 1.2. Perfect!

It's a dumb load and dumb loads do not radiate.

Now that you have replaced the coax with an appropriate size for the distance and frequency of interest, you can now see your crappy antenna without all that coax loss. Ground clamps have nothing to do with it.

[K5LXP]

I show 50ft of RG58 has about 4dB of loss on 2M.  LMR400 has about 1dB so the difference is about a 50% improvement in transfer.  In terms of dB that's not going to be a remarkable signal difference - half an S-unit.  The match is another story though, need to sweep the antenna and figure out what the issue there is.  A "bad" antenna can account for a much greater degradation of signal than some coax loss.

One of my V/U FM antennas is fed with 75 feet of RG8X.  I use it for local repeaters and simplex.  Typical distances are under 10 miles and anything I hear is "DFQ".  Just pointing out to AI5BC that sometimes good enough is good enough, if I had 75' of heliax to that antenna it would net me no operational improvement.

Mark K5LXP
Albuquerque, NM

[AC2EU]

My guess is that the antenna is not resonant in the 2 meter band or not 50 ohms at resonance. This mismatch will cause different impedance at the shack end depending upon the velocity factor of the cable and it's overall length.

You have changed TWO of the variables. The VF of RG58 is 0.66, while the VF of LM400 is 0.84. You also changed the length.
The safest way to ensure that the shack will see the antenna impedance -even if it's not 50 ohms, is to use multiples of a half wave length.
However if you antenna is 90-j100, you will still have a high SWR, so make sure the antenna is tuned by using cable that is some multiple of a half wave for the velocity factor.

For 146 Mhz;  free space half wave is 38.46 inches
                     for RG 58 it is 38.46 * .66 = 25.38 inches
                     for LM400 it is 38.48 *.84 =32.31 inches

Try trimming the cable some multiple of 32.31 inches , then see what you have. You may have to tune the antenna as well.

The original cable and length may have serendipitous transformed the impedance of a untuned antenna!

[WB6BYU]

The original cable and length may have serendipitous transformed the impedance of a untuned antenna!

Actually, while the cable length and/or velocity factor does
change the impedance of the antenna at the feedpoint, it
can't change the SWR if the cable impedance matches your
SWR meter.  (There is still the reduction in SWR with length
due to cable losses, of course.)

Basically, the impedance just runs circles around the Smith
chart as you change the cable length, but doesn't get any
closer to the center (lower SWR).

Where you can get that transformation effect is when you
use a feedline of a different impedance - a common example
is a 75 ohm matching section:  changing that length will
vary the SWR (relative to 50 ohms) seen at the radio, but
changing the length of 50 ohm cable between the matching
section and the radio won't.


If you do see a change in SWR (relative to 50 ohms) when you
change the length of a 50 ohm feedline to the radio, then
you probably have common mode current, so the outside of
the coax is effectively part of the antenna.  In that case, you
have actually changed the antenna length, so it isn't
surprising that the SWR changes.

[AI5BC]

One of my V/U FM antennas is fed with 75 feet of RG8X.  I use it for local repeaters and simplex.  Typical distances are under 10 miles and anything I hear is "DFQ".  Just pointing out to AI5BC that sometimes good enough is good enough, if I had 75' of heliax to that antenna it would net me no operational improvement.

I agree 100%. With 75-feet of RG8X, the Return Loss is -10dB. With no antenna connected your transmitter and the ham radio operator is happy with a SWR = 2. It leaks enough so you can operate repeaters and close by mobiles with it. No antenna required.

[K5LXP]

With no antenna connected your transmitter and the ham radio operator is happy with a SWR = 2. It leaks enough so you can operate repeaters and close by mobiles with it. No antenna required.

Who knew.  Here I've been putting antennas up for decades when all I needed was an open coax.  Just think of all the money I would've saved putting up a stupid tower and beams.  I'm going to sell my SWR meters too, if they're never going to read over 2:1 I won't have to worry about SWR ever again! 

Mark K5LXP
Albuquerque, NM

[WB6BYU]

...
With 75-feet of RG8X, the Return Loss is -10dB. With no antenna connected your transmitter and the ham radio operator is happy with a SWR = 2...

With RG-58 perhaps, but not with RG-8X (unless you are
using a pretty poor type).

With Belden 9258 (RG-8X), the return loss is 5.6 dB and
the SWR is 3 : 1 with the far end open.

[AI5BC]

With RG-58 perhaps, but not with RG-8X (unless you are
using a pretty poor type).

Exactly, we are talking about hams, they buy the cheapest crap they can. You are correct there is real RG8X like 9258 for $2/ft or sammy hammy grade for 65-cents/ft. 

[AI5BC]

I'm going to sell my SWR meters too, if they're never going to read over 2:1 I won't have to worry about SWR ever again!

You would not have any use for them. With a coax RL = -10 dB, your SWR meter will read 2 no matter what is terminated on the end of the coax, open, short, load or antenna will always read 2.  Besides you know the point I am making. At VHF and UHF most hams have undersized coax and would never know if they have a functional antenna or not. That is what the OP just discovered. He finally replaced an inadequate coax and can now see his crappy antenna he was happy with operating a repeater 10 miles away. You can do that with a paper clip or open coax.

[K5LXP]

With a coax RL = -10 dB,

Why would 50 ohm coax have a 10dB return loss?

your SWR meter will read 2 no matter what is terminated on the end of the coax, open, short, load or antenna will always read 2.

With 50 ohm line and a 50 ohm load, it would be 1:1, >25dB RL.

Mark K5LXP
Albuquerque, NM

[AI5BC]

Why would 50 ohm coax have a 10dB return loss?

Is this a trick question? You know the answer. If your coax insertion loss is 5 dB, then your coax RL = -10 dB all day long 7 x 24 x 365

your SWR meter will read 2 no matter what is terminated on the end of the coax, open, short, load or antenna will always read 2.

With 50 ohm line and a 50 ohm load, it would be 1:1, >25dB RL.[/quote]
OK my bad typo. Assuming the coax is in good repair, if you terminate the coax with a precision 50-ohm load, the RL will improve to -25 dB or better, about a SWR = 1.2

That is the whole point, if you have inadequate coax with too high of insertion loss, the CB SWR meter is worthless. If the coax RL = =10 dB, you will never read anything higher than SWR = 2. It is dumbed down to a range of 1 to 2. Worthless. Under those conditions, if the SWR budges means you have an antenna problem.

Today commercial operators have eliminated the coax loss problem. The radios are not in the equipment shelters. They are mounted tower top with the antennas. MW radios are bolted directly to the feedhorn, and UHF radios use a short 1-to-2-meter length 1/2 heliax resulting in a RL = less than 1. If the SWR shoots up to 1.5, tower crew rolls within 24 hours.