Difference in ability to TX and RX on base station

[nokones]

You need to ascertain the tuned center frequency of the Antenna. Your antenna requires tuning. If your VSWR is 1.2 at 462 Megs and 1.8 at 467 megs it is obviously that your antenna needs to be snipped some more if your desire is only talk to repeaters. If you desire to transmit to both mobiles and repeaters you might want to consider tuning for 465 megs.

[SteveShannon]

46 minutes ago, Whiskey363 said:

Steve,

  With that being said what would you estimate the actual SWR reading is? From my understanding anything over 2 can damage the radio. Or is the incorrect SWR reading at the base of the radio taken into account with the 2 or lower number?

Most modern radios can withstand an SWR well over 2.0:1 without damage.  You can calculate the SWR at the antenna based on the losses you have calculated. Your SWR meter calculated 1.8:1 with the lossy cable, but that doesn’t include the losses for both forward power and reflected power. You calculated that 63% of the power is attenuated.  I’m going to use 100 watts as the RF output just to make the calculations easier. With a 50 watts transmitter the values would be half.

Your radio transmits 100 watts.  You calculated 63% loss so 37 watts arrives at the antenna. So, this is why the measurement location matters.  At the radio Forward Power is 100 watts and by the time it gets to the antenna Forward Power has been attenuated down to 37 watts.

Due to an impedance mismatch some portion of the Forward Power reflects from the feedpoint and heads back towards the radio. That’s the Reflected Power. Again there’s a 63% loss before it arrives at the radio. Loss is loss for both Forward and Reflected power.

Here’s an interesting article about SWR that correlates reflected power percentage to SWR in a table: https://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdf

You measured an SWR of 1.8:1 (I assumed at the radio).  That correlates to 8.2% reflected power. So, at your hypothetical radio, when transmitting 100 watts forward, 100 watts was seen as the Forward Power and 8.2 watts was seen as the Reflected Power and an SWR of 1.8 was calculated from those two values.

But we know that 8.2 watts is the Reflected Power measurement after losing 63%, so 8.2 watts is really 37% of the actual Reflected Power (call that RP). As an equation 8.2 = 0.37 * RP. 
Rearranging, RP = 8.2/0.37.  Solving, RP is 22.16 watts actually reflected at the feedpoint.

But only 37 watts even made it to the feedpoint.  So an accurate calculation of SWR at the feedpoint would be based on the Forward Power of 37 watts and Reflected Power of 22.16 watts. Again looking at the table in the ARRL article, the reflected power percentage is 22.16/37 or 59.9% and that calculates to an SWR of 7.85:1

(I used the online SWR calculator at https://www.aareff.com/en/calculating-swr/?srsltid=AfmBOoqd-GzPr7AeD9xpbmFecxHoSzzTGyKbeLlR00b78uweI9rYUxED)

 

That doesn’t seem right to me if you’re using an antenna tuned for GMRS frequencies.  Please check my math.  

[WRUE951]

1 hour ago, Whiskey363 said:

I'm using the Tram 1486, but was thinking of switching to the Comet CA-712EFC 460-470MHz 9 dBi Gain Base/Repeater Antenna when they come back in stock. 

DX has them in stock   https://www.dxengineering.com/parts/cma-ca-712efc   and GigaParts  https://www.gigaparts.com/comet-ca-712efc-460-470mhz-base-repeater-antenna.html

[WRUE951]

1 hour ago, SteveShannon said:

One thing to be aware of is that SWR will appear to go up, possibly by quite a bit. Here’s why:

this is so true,,,  i run 50' of LMR 400 and fought a poor SWR issue..  My antenna on a separate 25' LMR test lead was hitting 1:2-1:3 on my repeater ch.. but 3:1 on my mounted 50'  antenna lead, which was out of the same section of my test lead.. Where i 45'ed into my wall i thought it was a nice sweeping 45.  As i pulled the bend outward to get a wider arc the SWR dropped to normal.  Ended up creating like a long drip loop to avoid the 45 bend..  

 

[Whiskey363]

4 hours ago, LeoG said:

This is where I got mine.  And the paperwork says 9.8dBi gain.

Thank you for the link. Are you satisfied with the performance of the antenna?

[Whiskey363]

4 hours ago, nokones said:

You need to ascertain the tuned center frequency of the Antenna. Your antenna requires tuning. If your VSWR is 1.2 at 462 Megs and 1.8 at 467 megs it is obviously that your antenna needs to be snipped some more if your desire is only talk to repeaters. If you desire to transmit to both mobiles and repeaters you might want to consider tuning for 465 megs.

Being new to this I paid the seller I purchased from to trim it to "GMRS frequencies" so I would not have to deal with this, but the articles I read stated what you did. That a higher SWR on lower frequencies meant the antenna was to short and a higher SWR on higher frequencies meant the antenna was to long. When I get the new coax I suppose I will have to study how to trim the antenna, and by how much. 

[SteveShannon]

34 minutes ago, Whiskey363 said:

Being new to this I paid the seller I purchased from to trim it to "GMRS frequencies" so I would not have to deal with this, but the articles I read stated what you did. That a higher SWR on lower frequencies meant the antenna was to short and a higher SWR on higher frequencies meant the antenna was to long. When I get the new coax I suppose I will have to study how to trim the antenna, and by how much. 

If you can borrow an antenna analyzer capable of UHF hook it directly to the antenna. Then tune the antenna to the frequency you want. Tune it before adding coax. Coax will just make it look better unless the coax is damaged or the wrong impedance. 

[Whiskey363]

Great idea, thank you!

[SteveShannon]

7 minutes ago, Whiskey363 said:

Great idea, thank you!

By the way, here’s a video about tuning the Tram 1486:

 

[Whiskey363]

Great concise video, thank you for sharing. Could have saved the $40 and cut it to 465 from the start. 

[SteveShannon]

3 minutes ago, Whiskey363 said:

Great concise video, thank you for sharing. Could have saved the $40 and cut it to 465 from the start. 

You can still check it to see how it is cut. 

[Whiskey363]

1 hour ago, SteveShannon said:

You can still check it to see how it is cut. 

That's the plan, just waiting for the new coax!

[SteveShannon]

Just now, Whiskey363 said:

That's the plan, just waiting for the new coax!

I meant physically measure the element and compare it to the length mentioned in the video. 

[Whiskey363]

25 minutes ago, SteveShannon said:

I meant physically measure the element and compare it to the length mentioned in the video. 

Steve, that is the plan, I am just waiting for the coax so I don't have to take down the antenna twice.

[SteveShannon]

1 hour ago, Whiskey363 said:

Steve, that is the plan, I am just waiting for the coax so I don't have to take down the antenna twice.

Good thinking!