Firestik MURS45 / Wouxon KG805M SWR

[fremont]

Used a fender mount (with NMO adapter and SS spring) to install Firestik MURS45 on truck.  Attached to Wouxon KG805M and used a Surecomm SW-33 meter.  Got reading of 19.9 SWR on "Blue" (154.570).  Would you chalk this up to "HTs give wacky readings*," lack of ground plane, etc?  (* I love Surecomm's instructions, e.g., hold vertical, place BOTH hands on meter then depress PTT...... feels like ham Twister.)

[marcspaz]

Hmm..  odd, why did you vote down my response? You don't believe adding the spring can cause problems? 

I noticed @WRAM370 gave the same advice and you agreed to try removing it. 

[mbrun]

Something in the mix sounds defective. Defective/incorrect antenna, defective mount, open circuit or short circuit in the RF path, defective meter, etc…

I am not familiar with the antenna referenced, but it does not appear it needs to be or can be tuned from what I see at one website I looked. And WOW! The antenna manufacturer is boasting 400 watt capability for a service that is only allowed 2. Very interesting.


Michael
WRHS965
KE8PLM

[marcspaz]

Are you using a grounding plate/shield to stop rf from the antenna soaking the meter?

[wayoverthere]

Given it's a mobile antenna, rather than a handheld, and an swr that high, I lean more toward either a ground plane issue or a short in the cable.

Handhelds are definitely wonky with handheld antennas (again, ground plane), but it shouldnt matter for a mobile antenna; it should be getting it's ground plane via the mount, not the ht.

[fremont]

4 hours ago, marcspaz said:

Are you using a grounding plate/shield to stop rf from the antenna soaking the meter?

No.  I was sitting in the driver's seat with the antenna 4' away.  What's best way to operationalize your recommendation?

[marcspaz]

18 minutes ago, fremont said:

No.  I was sitting in the driver's seat with the antenna 4' away.  What's best way to operationalize your recommendation?

 

The meter should have come with a ground plate for testing HT's with antennas.  I though the meter was at the base of the antenna.  If you are 4 feet away, especially if the meter is inside the vehicle, that should be plenty of distance and shielding. 

[wayoverthere]

54 minutes ago, marcspaz said:

 

The meter should have come with a ground plate for testing HT's with antennas.  I though the meter was at the base of the antenna.  If you are 4 feet away, especially if the meter is inside the vehicle, that should be plenty of distance and shielding. 

incidentally, i have both the SW33 (the small inexpensive one) and the SW102 (the slightly larger, slightly more money, full display model).  The SW33 did not come with a plate, though i don't think the enclosure being solid metal hurts...it DID include a 5 watt rated dummy load. The SW102 included the plate.

I did a couple quick tests to see if RF affected the readings, though i used GMRS since i don't have anything approved for MURS.  With both meters and my KG805g HT, I used channels RP21 (467.700 tx) and simplex ch 3 (462. something...the number slips my mind at this second). SW33 said 1.05 and 1.02, while the SW102 showed 1.0 for both. Then, I freed up some slack in the cable, and held the HT up within a foot of the antenna, and tested again with the SW33.  Same 1.02 on Ch 3, RP21 actually dropped slightly, to 1.03.  At least for UHF, it seems pretty unaffected...I can try to retest with a different handheld and VHF this weekend..will need to switch antennas and try messing with the ground plane and see if goes that bad.

[fremont]

2 hours ago, marcspaz said:

 

The meter should have come with a ground plate for testing HT's with antennas.  I though the meter was at the base of the antenna.  If you are 4 feet away, especially if the meter is inside the vehicle, that should be plenty of distance and shielding. 

I screw the SW-33 meter (using SMA-F adapter) into the top of the HT then connect coax to the top of the meter.

[fremont]

1 hour ago, wayoverthere said:

I did a couple quick tests to see if RF affected the readings, though i used GMRS since i don't have anything approved for MURS.  With both meters and my KG805g HT, I used channels RP21 (467.700 tx) and simplex ch 3 (462. something...the number slips my mind at this second). SW33 said 1.05 and 1.02, while the SW102 showed 1.0 for both. Then, I freed up some slack in the cable, and held the HT up within a foot of the antenna, and tested again with the SW33.  Same 1.02 on Ch 3, RP21 actually dropped slightly, to 1.03.  At least for UHF, it seems pretty unaffected...I can try to retest with a different handheld and VHF this weekend..will need to switch antennas and try messing with the ground plane and see if goes that bad.

What's your antenna setup?

[wayoverthere]

1 hour ago, fremont said:

What's your antenna setup?

It's a Midland 5/8 wave whip, mag mounted on a 12"x8" piece of sheet metal and tucked on top of a bookcase. Not sure how well the current placement will do for simplex, but it does fine for repeater work

[jgillaspy]

Make sure you have the SWR meter connected correctly. I had this issue when I had the meter connected backwards. . . Really made me feel stupid when I realized it later. I lost several hours of troubleshooting to my mistake. Mine read 19.9 as well. The meter ports are labeled - verify your connections.

JG

[fremont]

2 hours ago, jgillaspy said:

Make sure you have the SWR meter connected correctly. I had this issue when I had the meter connected backwards. . . Really made me feel stupid when I realized it later. I lost several hours of troubleshooting to my mistake. Mine read 19.9 as well. The meter ports are labeled - verify your connections.

JG

I could easily do that.....but this time I had it right.  Weird how the readings are the same, though.

[wayoverthere]

58 minutes ago, fremont said:

I could easily do that.....but this time I had it right.  Weird how the readings are the same, though.

I think 19.9 is just the "max" it can display...I got it a couple times checking handheld antennas with no ground plane.

Touching the connectors changed the numbers noticeably, depending how and which ones I touched.

[fremont]

5 minutes ago, wayoverthere said:

I think 19.9 is just the "max" it can display

Yikes!

[fremont]

This question popped into my head:  Much (all?) of Firestik's product line uses 3/8x24 threaded connectors.  For their CB antennas, most use a 1/2" opening mount to which a Firestik (or other) 3/8x24 connector is secured.  The coax uses ring terminals for the connection.  So, this is different than my NMO setup.  I bought an adapter which is NMO-to-3/8x24 threaded.  The adapter sits atop of a rubber gasket which is resting on my fender mount.  There is no metal-to-metal contact as far as the antenna assembly and the mount.  Now, the same is true for my ghost GMRS antenna with no apparent issue, but it made me wonder if a lack of metal-to-metal decreases a ground plane's effectiveness.

[marcspaz]

There is a nylon spacer between the mount and the center feed on the 3/8x24.  Make sure its aligned correctly and not crushed.  If you have an ohm meter, disconnect the cable from the radio and the antenna from the mount and see if the center lead is shorted to ground.

[WRAM370]

You mentioned you were using a SS spring. It this a typical SS spring that is used on 102” CB whip antennas? If so, that may be impacting the SWR as it may be coupling to the 3/8” mount of the antenna and electrically lengthening it. Just a thought.

[marcspaz]

Oh, man!  I missed the spring part. Adjusting the antenna length a couple of millimeters can throw off the SWR.  Adding a 4 or 5 inch spring will definitely through it off. 

[fremont]

1 hour ago, WRAM370 said:

You mentioned you were using a SS spring. It this a typical SS spring that is used on 102” CB whip antennas? If so, that may be impacting the SWR as it may be coupling to the 3/8” mount of the antenna and electrically lengthening it. Just a thought.

Yes!  Probably 4".  Will remove and report back.

[fremont]

[fremont]

Quick update

On 1/20/2022 at 12:24 PM, marcspaz said:

If you have an ohm meter, disconnect the cable from the radio and the antenna from the mount and see if the center lead is shorted to ground.

Checked this and center lead is same as mount (< 1 ohm).

[mbrun]

Quick update
Checked this and center lead is same as mount (

Not clear. Are you measuring this between the center lead of the coax and the exposed metal portion of the mount? If possible, could you post a picture that depicts where the ohm meter as connected? Want to make sure we are all on the correct page.


Michael
WRHS965
KE8PLM

[fremont]

11 minutes ago, mbrun said:

Are you measuring this between the center lead of the coax and the exposed metal portion of the mount?

Now I'm confused. ;o)

I measured at 1) the NMO's center lead (which contacts the spring-loaded "button" within antenna's base); 2) the NMO outer nut, and 3) the fender mount itself.  All returned essentially the same reading.  As a check, I compared these readings with a ground strap from my firewall to the engire block--same reading.

Apologies if I'm still missing it.

[mbrun]

Now I'm confused. ;o)
I measured at 1) the NMO's center lead (which contacts the spring-loaded "button" within antenna's base); 2) the NMO outer nut, and 3) the fender mount itself.  All returned essentially the same reading.  As a check, I compared these readings with a ground strap from my firewall to the engire block--same reading.
Apologies if I'm still missing it.

If I am interpreting you correctly, it sounds like there is a dead short between the center connector of your NMO and the exposed metal portions of the NMO and vehicle metal. If that is what you are conveying, that is not good. You will need to track down where that short is occurring.


Michael
WRHS965
KE8PLM

[marcspaz]

36 minutes ago, mbrun said:

If I am interpreting you correctly, it sounds like there is a dead short between the center connector of your NMO and the exposed metal portions of the NMO and vehicle metal. If that is what you are conveying, that is not good. You will need to track down where that short is occurring.


Michael
WRHS965
KE8PLM

 

@fremont  I am in agreement with @mbrun.  If the cable is NOT connected to the radio nor an antenna, and you measure between the center lead of the NMO and ground (and/or the antenna mount bracket), and it shows <1 ohm, then you have a dead short in the cable or the mount.

 

Take the coax completely off the mount and see if the short between the center lead and the outside of the cable still exist.  If it does, you need a new cable.  If it doesn't, its the mount, and I would still be very suspicious of the nylon isolator.