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[WA3SKN]

What test eqpt do you have available?

-Mike.

[W5OT]

Sorry for the late update, I've been out of town on a job.

I checked the antenna with my antenna analyzer and plotted the SWR curve. 1:1 centered in the 2M band @ 146MHz. Nice sharp dip rising to 1.5 @ 145/147 MHz and gets worst the farther away from 146.  My radio folds back an anything 1.5 or greater.  Removed the tower grounding block at the base of the tower, no change. Removed the 12ft extension from the grounding block to the radio, no change. 

So, it appears (as AI5 stated) that my Diamond F23h antenna "is crap".  Couldn't find any documentation on the stated bandwidth of this antenna. Only thing I know is it's single band (tunable), high power (350W), and 3-5/8 wavelength with a gain of 7.8.

Now, the question.  Put the RG-58 back up and live with a 2dB loss at the antenna or go with a different antenna and stick with the LMR-400.  (does the antenna gain make up for the apparent coax losses?). I do have a 1/4 wave ground plane 2M vertical antenna laying around, but I have no idea it's BW.  Nothing in Diamonds catalog has BW specs on any of their antennas and I wouldn't mind some recommendations for a general purpose vertical with broad BW for 2M work.

[AC2EU]

I built a cheezy 2 meter quarter wave antenna with a 10 AWG soldered to the center pin and 4 radials connected to the 4 holes on the SO239 bulkhead type.
It worked a lot better that what you have!
I did learn that the radial angle made quite a difference with antenna impedance.
What is the impedance of YOUR antenna at resonance?

[WB6BYU]

...I do have a 1/4 wave ground plane 2M vertical antenna laying around, but I have no idea it's BW...

A quick model of a 2m ground plane with 4 sloping radials made
from #12 wire gives an SWR bandwidth of about 9 MHz at 1.5 : 1.
Using fatter radiators will improve that number somewhat.

Sloping the radials is important to get the minimum SWR close
to 1 : 1.  But the bandwidth should be sufficient for your needs.

[WB8VLC]

The F23 is described as a triple 5/8th wave phased vertical so it reasons that it would be a narrower bandwidth design. 

I don't have my modelling program on this PC to look at such an antenna but it wouldn't be unreasonable to have only a useable 2 to 3 MHz bandwidth at most.

[K5LXP]

First question is how much is the radio cutting back at the band edges?  And, is that at all power settings or just full power?  Point being, maybe at 50W it might not cut back as much, and you wouldn't have to compromise RX through a lossy feed.

Another option would be to measure the R+Xj and switch in an inline matching section.  You'd still have the match loss but the radio would be "happy".

Mark K5LXP
Albuquerque,NM

[WB6BYU]

I don't have my modelling program on this PC to look at such an antenna but it wouldn't be unreasonable to have only a useable 2 to 3 MHz bandwidth at most.

I have the spec sheet here for the Diamond X-50/200/300/500
antennas.  The shortest X-50 has an SWR bandwidth at 1.5 : 1
of at least 6 MHz.  As the gain increases with the longer versions,
the SWR curve gets sharper:  for the longest X-500 model it
is about 2.5 MHz (but still less than 2 : 1 across the band).

That's as expected with the elements connected in series.
And it isn't uncommon to see some narrowing due to the
dual-band operation as well.

If you want a wide bandwidth, consider a stacked array of
folded dipole elements.

Otherwise, there are several options.  One is to center it
in the part of the band where you do you do most of your
operating, or at least where you need the highest power.
You can see if the radio still shuts back as much when
you switch it to lower power - that may  be perfectly
adequate for many local repeaters.

You can tune it for one end of the FM band and switch in
a matching circuit (likely just one coil or capacitor, or a
shorted piece of coax) to match it at the other end of
the band.  Between the two you should be able to operate
the whole band.

It may be possible to design a matching circuit to go at
the antenna to increase the bandwidth without adding
significant losses, but that will depend on the actual
impedances measured at the antenna.

[WA3SKN]

Have you done any cutting or trimming of any of the elements yet as per the instruction sheet?
The sheet indicates a 1.5:1 SWR, but you should be able to bring it down a bit... and there is also a matching section that can be adjusted.
The antenna is three 5/8 wavelengths with a matching section and for 2m only operations.  Coax loss is never recovered.

-Mike.

[W5OT]

Have you done any cutting or trimming of any of the elements yet as per the instruction sheet?

Cut and trimmed EXACTLY per the instructions for 146MHz. It also reads 1:1 at that freq on my antenna analyzer.  The radio is an Icom 746Pro and at full power (100watts), it's folding back about 50% once the SWR reaches 1.5 @ +/- 1 MHZ from 146.  Not that I need that much power to reach the 20+ repeaters in the Tucson area and EAARS/Rimlink networks, but some of those in the adjoining counties are hit and miss.  It appears that the F23h is an excellent antenna for a repeater on a fixed freq pair, but not so well suited for wide coverage of the entire 2 meter band.  I think I'm going to go with the 1/4 wave ground plane and the LMR-400 coax.  A wide bandwidth is more preferable than the higher gain and I'd rather have the power doing useful work than heating my coax.  Thanks everyone for your input.