Terminal or Antenna

[gothmog05]

New to GMRS and have a SWR question. I instaled a wouxun kg-1000g in my truck. I have been slowly checking everything along the way to make sure everything still works. I am using a midland mag mount with a midland 6 db antenna. With everything installed i was getting a 1.6 swr using a surecom sw-33 meter. The stock cable length was way to long so i cut about 15ft off leaving 3ft. After this i rechecked the swr and was getting 2.8 to 3.0 swr. So my question is am i needing to tune the antenna, or did i do a crappy job with my pl259 connector?

 

Thanks 

[Lscott]

New to GMRS and have a SWR question. I instaled a wouxun kg-1000g in my truck. I have been slowly checking everything along the way to make sure everything still works. I am using a midland mag mount with a midland 6 db antenna. With everything installed i was getting a 1.6 swr using a surecom sw-33 meter. The stock cable length was way to long so i cut about 15ft off leaving 3ft. After this i rechecked the swr and was getting 2.8 to 3.0 swr. So my question is am i needing to tune the antenna, or did i do a crappy job with my pl259 connector?

 

Thanks 

Looks like you need to trim your antenna. The question is it too long or too short. If the antenna provides a means to adjust the length a bit try that first.

 

As far as the SWR going up after cutting the coax length down a bit isn't surprising. The SWR depends on how much power is output by the transmitter and what fraction of that at the end of the coax is reflected back by the antenna. A longer length of coax results in less power reaching the antenna and the reflected power, as a fixed function of the antenna match itself, is further attenuated by the extra coax length. The end result is a lower SWR as seen at the transmitter end.

 

[gothmog05]

Thanks for the reply. There is two set screws on my mount, i removed the antenna and screwed the lower set screw in all the way so that the antenna would rest on it and lengthen it a little. SWR dropped so looks like i am going to have to find a new antenna.

[mbrun]

New to GMRS and have a SWR question. I instaled a wouxun kg-1000g in my truck. I have been slowly checking everything along the way to make sure everything still works. I am using a midland mag mount with a midland 6 db antenna. With everything installed i was getting a 1.6 swr using a surecom sw-33 meter. The stock cable length was way to long so i cut about 15ft off leaving 3ft. After this i rechecked the swr and was getting 2.8 to 3.0 swr. So my question is am i needing to tune the antenna, or did i do a crappy job with my pl259 connector?

 

Thanks

I must admit I am questioning the quality of the PL259 connection, assuming absolutely nothing else changed besides shorting the cable. That amount of SWR change with removal of just 15 foot of cable strikes me as excessive. Honestly, I would have expected it to go up a tad, maybe .1-.2, but your values nearly doubled.

 

I recommend trying your hand at replacing the connector one more time before trimming an otherwise pre-tuned antenna or investing in a new one.

 

 

Michael

WRHS965

KE8PLM

[gothmog05]

So i haven't done anything yet with the antenna but i am looking at getting the Browning BR-450 as a replacement. But regarding the connector maybe being bad is there a way for me to check it with a multimeter before replacing the connector?

[mbrun]

Never hurts to check with an ohm meter on the lowest range setting. You should have infinity ohms as measured between center conductor and shield at both ends of cable. You should have near zero ohms when measuring continuity of the center conductor end to end and similarly near zero ohms for the shield.

 

While the ohm meter is an appropriate level-1 test for the cable, the ohm meter is DC, where as RF is AC and AC resistance (impedance) is affected frequency and many other factors.

 

A pinch, a sharp bend or deformation of the cable, a scrap on the outside surface of the center conductor, a poor solder connection, a less then optimal crimp connection are all candidates for altering the impedance of the feed-line and thus the SWR of the antenna.

 

Absent more sophisticated test equipment, some knowledge and a little trial and error and patience is ultimately required.

 

I cannot tell you how many times I have seen the numbers change based on changing a connector, swapping jumpers or the adapters used for the measurement. Sometimes something as simple as disconnecting, reconnecting and tightening the connection can yield positive results.

 

 

Michael

WRHS965

KE8PLM

[gothmog05]

Replaced connector, with the same results. 2.8 to 3.0. This time i only soldered the pin and didn't solder the shield to the plug. I read that soldering the shield can result in overheating the insulator in the wire. Also with the new connector i extended the antenna as such as i could in the mount, about 1/4 of a inch. the results were a drop in swr to 2.0 - 2.2. already ordered a new antenna but i only have the two connectors so cant try another one without getting more.

 

Thanks for all the help.

 

Justin

WRKX762

[OldRadioGuy]

If the antenna was anything close to a match to start with the 50 ohm coax length should have little to no impact on SWR.

I would assume it's the connector and try doing it again.

 

For those of you familiar with the Smith Chart....

If you have a load that is at or near the center of the chart, 50 Ohm coax just rotates it around the center point.

So the SWR should not change appreciably.

 

I can't imagine that the coax would have been part of the matching scheme because it would have to be other than 50 Ohms to make a 1/4 wave transformer.

I think that would be highly unlikely.

 

Vince

[maddogrecurve]

On 4/4/2021 at 2:04 PM, gothmog05 said:

New to GMRS and have a SWR question. I instaled a wouxun kg-1000g in my truck. I have been slowly checking everything along the way to make sure everything still works. I am using a midland mag mount with a midland 6 db antenna. With everything installed i was getting a 1.6 swr using a surecom sw-33 meter. The stock cable length was way to long so i cut about 15ft off leaving 3ft. After this i rechecked the swr and was getting 2.8 to 3.0 swr. So my question is am i needing to tune the antenna, or did i do a crappy job with my pl259 connector?

 

Thanks 

Gothmog05,

 

We have to remember that the coax becomes part of the antenna!  I do not understand it all really but the length of the coaxial cable does make a difference, but probably not as much as what you saw going on.  I had a recent issue with SWR as well on a new antenna install, I put a new PL259 connector on my RG8 coax (I have put many connectors on) but the PL259 was not the right size for the RG8 so I had struggled to make a good solder joint on the center pin.  I could not get the antenna tuned well at all and ended up cutting too much off of it for GMRS!

Long story short, I put a new connector on, and ended up trimming the antenna further down so that it resonates on Murs channels for that radio LOL>

Did your terminal fit the coax properly?

[BoxCar]

Antennas often use the shielding of the coax as part of their ground or counter-poise. It's possible you trimmed too much and affected the loading of the antenna. Assuming it's not a bad connector, an antenna analyzer (vector network analyzer) would answer the question. VNAs can be found on Amazon. I recommend one with the large screen for better clarity.

[WRNA236]

The antenna is almost always not supposed to use the coax shield, that would not make it not a shield anymore.  You go to great lengths to prevent this in fact with chokes.  Coax as feedline should not be radiating and the length should have no significant effect on SWR.

There are intentional exceptions to this, such as using coax stubs as impedance matching and there are designed coax antennas.

Usually when this will happens it's due to an issue such as the mount location not providing a sufficient ground plane (counterpoise).  The RF wants to complete the circuit and will find whatever return it can back to the radio.  This would be my guess as to your issue.  The mount isn't well grounded to the vehicle or you're using an antenna that needs a plane and you don't have one. 

You might make it work but it's not optimal.  For one the coax shield radiating will create local EMI.  This causes anything from irritating noise to potentially problems.  You might just get buzzing on your mic or you might reset your computers.  On the flip side if the shield is part of the antenna you open yourself to conducting radiation from the environment, you'll get bad alternator noise or cross talk from other radios or your cell phone. 

Shields work both way, to keep transmitted energy contained within the coax and to keep external noise out.  So in a vehicle having coax radiating is almost always bad.

BTW, regards to soldering PL259, it's kind of an art because heat will definitely melt the dielectric so it's about speed.  You want a good soldering iron with lots of mass.  You want the heat put into the outer shell fast, get the braid soldered in a second or two and get out.  If you have to sit there and hold your iron for tens of seconds then you're probably ruining the dielectric.  I have an old 100 watt leaded glass soldering iron I use for this although honestly I prefer crimp connectors much more.  In the case of PL259, though, I use solder center, crimp outer types.  I find the crimp/crimp have too long of a nose to always fit SO239s.  But any other type - BNC, N, SMA - I always crimp.