VHF mobile antenna issue.

[vdubb16]

Originally i installed my 2m and attached a RS window mount scanner antenna. The element half was double side adheared to the base section and then adheared the same fashion to the roof as a temporary setup while i waited on my NMO cable and 1/4 wave antenex QWB152.

Antenex QWB152 [QWB152] - $12.95 : The Antenna Farm :: , Your Two Way Radio Source!


Larsen NMOKUD [NMOKUD] - $12.95 : The Antenna Farm :: , Your Two Way Radio Source!

The scanner antenna matched down to 2.0-2.1. on low power. i didnt plan on tx'n much till i got my 2m antenna. but i always check anyways for damage. I had excellent receive. it was hit or miss at 40 miles or so but crossing a bridge it with no overhead lines or other interference the scanner antenna worked quite well. and tx on low power i could hit our fire repeater from about 20miles when im in the pva at work.

now that i installed the nmo mount and 1/4 wave. i struggle to tx/ rx the repeater within 5 miles. swr are down to 1:2 on low power. I have to switch to high power to even hit the repeater 90% of the time. even within 5 miles.

any suggestions as to why the antenna not designed for my use works much better than the one thats intended for this use?

tx:158.9550 rx:154.9950

 

[ve9jmc]

first thing i would do, even though it may sound stupid is to take a Multi Meter and do a continuity test betweern the center conductor of the coax and the shield just to ensure that there is no short in the coax or the mount. as well do a continuity test on the center pin of the coax connector to the antenna 1/4 wave element as to whether you get a dead short or if it's open.

hope that gives you some place to start. even though it sounds stupid.

 

[jim202]

first thing i would do, even though it may sound stupid is to take a Multi Meter and do a continuity test betweern the center conductor of the coax and the shield just to ensure that there is no short in the coax or the mount. as well do a continuity test on the center pin of the coax connector to the antenna 1/4 wave element as to whether you get a dead short or if it's open.

hope that gives you some place to start. even though it sounds stupid.

Even the best and long time techs make mistakes, or screw up. It is so easy to get one small strand of
the shield left on the center conductor insulation when you make up the RF connector at the radio. It
is better to check first before you solder or crimp the connector on. Has saved me some grief over
the some 45 plus years I have been doing this type of work.

 

[K9WG]

Just because you have a 1:2 VSWR doesn't mean you are radiating power from the antenna. What kind of coax are you using? A cheap coax can exhibit a good VSWR but have so much loss at VHF that little gets out. Also what is the length of your new install vs the older one. The more coax the more loss. Just some things to check as well as those mentioned above.

(Anymore I use RG6 satellite coax for my installs. Even though it is 75-ohm the quality of it makes up for the mismatch)

 

[LtDoc]

Have you tried tuning that new antenna? At 18 inches, I have a feeling that it's not exactly going to be very usable, too short.
- 'Doc

 

[rfradioconsult]

Wha type of VSWR meter are you using?

 

[vdubb16]

According to this calculator 18" is right on for the freq 154.995 which is my primary frequency of concern.
Antenna Length Calculation

The scanner antenna had 16ft of RSG58
the Larsen mount has 17ft of attached RG58A/U

The meter is also a radio shack model.
CB/High-Frequency Ham Power SWR Meter - RadioShack.com

I picked up a second one already to see if both showed the same results and they did. However not the greatest quality product. I have more faith in 2 of the same results, rather than one. however swrs do raise rather signifcantly on high power 2:5 - 2:7 at 75 watts. This is my first mobile setup so im inexperienced at what to expect. But seems rather high to me.

Im going to pull the 259 off and make sure i dont have any stray strand on outer shield where it shouldnt be and re solder another one on.

 

[W2NJS]

The theoretical quarter wave fo 154.995 mHz is 18.11 inches.

 

[davidgcet]

you need to double check that connector, if you overheated the dielectric during the soldering it will be
hosed as bad as if you had a strand of shield. also pull the NMO mount up and make sure it was not broken during install. pull the plastic cover back and look at the connection i have seen them break right at the center pin solder joint. i've also had them come in from the factory with a short!

the QWB152 does not require tuning, though you can cut it to really get it on freq. i doubt it is the problem, more likely it is the coax/connector.

your SWR goes up because of the drastic mismatch. keep in mind you are not using the best meter in the world to test, and it is likely not that evenly calibrated across the power levels. even a Bird 43 which is the most used meter in commercial comms is only really accurate at the top 1/3 or so of the scale, this is why it has so many different power level elements available for it. i have seen readings of 3 watts reflected with a 100W element and put in a 10W element and it changes to 6-7W reflected due to the higher accuracy. which would be about the same results as you are seeing so maybe your meter is tuned more for the higher thru power.

 

[rfradioconsult]

Your Radio Shack SWR meter is only rated for 3-30 MHz,not VHF. Unless you are using a meter specifically designed for VHF use you cannot depend on it giving you the correct reaading.

 

[davidgcet]

good catch, i did not even look at the specs for the meter.

 

[LtDoc]

Sorry, when you referred to installing a "2m" I assumed that meant an amateur 2 meter band radio for use. For operations around 155 Mhz, that antenna length should work.
- 'Doc

 

[vdubb16]

I was informed right after my last post via PM about the meters bandwith. i didnt realize that myself so all my talk about swr is out the window.

The radio is an Icom V8000, however im only using it to monitor public service. and for use along with my FD.

later down the road ill get my ticket and explore the 2m ham band.

I will replace the connector and see where it goes from there.

Even Wx hardly understood. tho its at the top of the antennas bandwidth. i thought i should hear something. being it was crystal clear with the other antenna.

 

[AK9R]

When you installed your NMO mount, did you scrape some paint off of the underneath side of the sheet metal around the hole so that the teeth on the bottom of the mount can make good contact with bare sheet metal?

 

[vdubb16]

When you installed your NMO mount, did you scrape some paint off of the underneath side of the sheet metal around the hole so that the teeth on the bottom of the mount can make good contact with bare sheet metal?

The answer to that is no. I removed the headliner and went down to a local shop and they drilled the hole and installed the mount for me. However i wondered if whatever coating thats on the metal should be removed from the interior side of the metal before i even had it installed, but assumed they'd know/do whats right.

 

[jim202]

inexperienced at what to expect. But seems rather high to me.

Im going to pull the 259 off and make sure i dont have any stray strand on outer shield where it shouldnt be and re solder another one on.

let me get this straight, your going to pull off the PL259??? How did you put it on in the first place?

You need to solder the shield to the connector shell to get a good electrical connection. I even end up filing
off the plating on the reducer and tinning it with solder before I assemble the connector.

Plus I also take my small long nose pliers and open up the holes in the outer shell. Take a file and remove
the plating where the holes are. Then when you assemble the connector, you fold the shield over the
reducer. Cut the length of the braid so it won't get into the threads. Screw it all together and then solder
the shield to the shell. Takes a rather hot and big iron to provide enough heat to make the solder flow.

Don't forget to solder the center wire in the center pin of the connector.

 

[vdubb16]

by clipping the end off and starting fresh was the thought

 

[davidgcet]

if you are going to cut it off, go ahead and cut the coax to length. i usually leave just a few inches of slack just for ease of neating the cables.

 

[vdubb16]

if you are going to cut it off, go ahead and cut the coax to length. i usually leave just a few inches of slack just for ease of neating the cables.

Thats what i did, 12ft of cable with the new PL-259, resoldered being more careful about the heat. and it works great now.

I appreciate the input fellas