High SWR when tuned to repeater channels?

[JFITZ7]

Hey everyone,

New to the radio world and I am in the process of tuning my mobile antenna and have a few questions. Currenlty running the BTECH 50X1 with a Bingfu mobile antenna. I've cut the antenna and have the SWR down to ~1.3 for simplex channels 1-22 but when I tune up RP15-RP22 my SWR goes up to 2.0. Any thoughts on what could be causing this?

[wayoverthere]

The short answer is that it's likely not quite short enough. The simplex channels that the 50x1 can access are all in 462.xxx. those repeater channels transmit in 467.xxx, which will want a little shorter antenna than 462, which requires a bit shorter than the ham 70cm band.

An analyzer like a nanovna would say for sure, though, where the "sweet spot" is at its current length.

[JFITZ7]

On 12/28/2021 at 11:40 PM, wayoverthere said:

The short answer is that it's likely not quite short enough. The simplex channels that the 50x1 can access are all in 462.xxx. those repeater channels transmit in 467.xxx, which will want a little shorter antenna than 462, which requires a bit shorter than the ham 70cm band.

An analyzer like a nanovna would say for sure, though, where the "sweet spot" is at its current length.

Thanks for the response! Just wanted to get another opinion before I kept cutting on the antenna. I'll give it a try and see if I can get it to come down.

[BoxCar]

Before you do any more to the antenna, CALCULATE what the length should be. There are several sites you can use that will provide an answer in either millimeters or inches. The adage measure twice, cut once always helps. The loading coil if used adds an even amount like a 1/4 or 1/2 wave so you need to be able to calculate the length of the stinger above the coil. The length is measured from where it comes out of the holder to the tip, not the physical length of the stinger so it needs to be a little longer than the calculated length.

[WRCZ387]

GMRS band center is 465 MHz [465.1375 if you're going to be precise]

Repeater inputs center is 467.6375 which is FRS channel 11

[OldRadioGuy]

The frequency difference we are talking about here is 1%.

So, would you want to shorten the antenna 1% ???? (rhetorical question, of course you wouldn't)

I just wonder if something else is going on.  Unless it's a very short antenna with a very narrowband match.  It just doesn't make sense. 

Maybe the meter is the problem.  CAn you try a different meter and cables?

Vince

[MichaelLAX]

17 minutes ago, OldRadioGuy said:

The frequency difference we are talking about here is 1%.

So, would you want to shorten the antenna 1% ???? (rhetorical question, of course you wouldn't)

1%? 

Where do you calculate that from?

[OldRadioGuy]

5/462 MHz x 100 = 1%  (rounded off)

The difference between the "lower channels" and the repeater channels is 5MHz. 

Vince

[MichaelLAX]

7 hours ago, OldRadioGuy said:

5/462 MHz x 100 = 1%  (rounded off)

The difference between the "lower channels" and the repeater channels is 5MHz. 

Vince

You want to drive from City A to City B in one hour; they are 100 miles apart.

You drive one half of the distance at 50 miles per hour.

What speed must you drive the remaining half to arrive within the one hour time limit?

A.  200 miles per hour

B.  100 miles per hour

C.  75 miles per hour

D.  Some maths are not linear!

Let's return to the situation at hand.

You point out (that the start of) the GMRS band is 462 MHz.

Now M is an abbreviation of Mega and Hz is an abbreviation of Hurts, so GMRS at 462 MHz really bad pain if you touch the wrong exposed wires! 

But in reality 462 MHz is 0.65 centimeters and 467 MHz is 0.64 centimeters, and antenna maths are not necessarily linear either! 

[AdmiralCochrane]

The math presented appears correct to me.  1% of 65cm is approximately  ¼".   Just a WAG, but I would present that the data provided suggests that either both SWR readings are showing a slightly longer than optimal antenna or possible difference in accuracy of the meter with different power output.  OP did not confirm that readings were taken with the same output.

This is one of the advantages of the nanoNVA tool vs a transmitter and wattmeter.  Without an analyzer, cut and see if the 1.3 goes up more than the 2.0 goes down, or if both go down. 2.0 SWR isn't going to hurt anything on GMRS, propagation will vary more with where you are standing, how straight up you hold the antenna (polarization is real) and how well charged your battery is. 

 

[OldRadioGuy]

Antennas are never a full wavelength so N/A.

Most simple vertical antennas are 1/4 wavelength. 

And it's spelled HERTZ not hurts.

Vince

[OldRadioGuy]

For those of you who don't believe it... let's do the 1/4 wavelength calculation.

300/(462MHz x 4) = 162.3mm  low channels

300/(467MHz x 4) = 160.6mm  repeater channels

(162.3-160.6)/162.3  x 100 = 1.04%

Looks like the math is linear enough.

Vince

 

[MichaelLAX]

43 minutes ago, OldRadioGuy said:

Looks like the math is linear enough.

Do the maths for me if I want to transmit 5 MHz apart on say: 25 MHz (12 meters) and 30 MHz (10 meters). Still want to shorten the antenna by the percentage difference?  Of course you wouldn't...

Remember, I was responding to your original post:

On 1/4/2022 at 8:38 AM, OldRadioGuy said:

The frequency difference we are talking about here is 1%.

So, would you want to shorten the antenna 1% ???? (rhetorical question, of course you wouldn't)

I just wonder if something else is going on.  Unless it's a very short antenna with a very narrowband match.  It just doesn't make sense...

I'm just pointing out that the 5MHz difference is not linear in antenna maths...

59 minutes ago, OldRadioGuy said:

And it's spelled HERTZ not hurts.

This emoticon, provided by this forum, "" is generally understood to be used to denote a joke...

Oh, OLDradioguy!  

[tweiss3]

Linear in wavelength, not linear in effect on the antenna SWR, especially many mobile antennas that have coils/capacitance in the bases.

[n4gix]

3 hours ago, OldRadioGuy said:

And it's spelled HERTZ not hurts.

I'm so old-fashioned that I still call it cycles! 

Rhetorical question of the day; would Hertz sue me for trademark infringement if I started a car rental and named it "Hurts Rent-a-Car?" 

[MichaelLAX]

On 1/6/2022 at 2:27 PM, n4gix said:

Rhetorical question of the day; would Hertz sue me for trademark infringement if I started a car rental and named it "Hurts Rent-a-Car?" 

YES!  Whether they would prevail at trial is another question, but do you have the resources to defend such an action?

Here in Los Angeles, we remember in the 80s, a discount fashion clothiers opening a small chain of stores named Sacks Fifth Off! (owned by David Sacks).

He was sued by Saks Fifth Avenue and entered into a settlement to stop using that trade name.

UPDATE: And a Chinese food restaurant in Studio City, City Wok, sued Universal City Studios, after it opened its massive retail shopping/dining facility on its property: City Walk and WON!  The judge ruled that it must always be referred to as Universal City Walk!

And some guy took Nipper and made him his very own California registered service mark (  )

[OldRadioGuy]

I can see that you are just a troll so I'll move on.

The "maths" speak for themselves. 

[WRKC935]

On 1/6/2022 at 2:10 PM, MichaelLAX said:

and 30 MHz (10 meters).

 

Since when is 10 Meters 30Mhz?  28.0 to 29.7 ain't 30. 

And as you DROP frequency antenna band width for a given band drops as well to the point that on 80 and 160 from the bottom to the top of the band changes the antenna length by several feet. 

Just sayin

[MichaelLAX]

1 hour ago, WRKC935 said:

Since when is 10 Meters 30Mhz? 

I didn't say 10 meters is 30 MHz; I said that 30 MHz is 10 meters!

As @OldRadioGuywould say:

Quote

The "maths" speak for themselves.

Actually 30 MHz is 9.99308 meters, to be more precise! 

I was just rounding up for you repeater guys! 

[n4gix]

On 1/7/2022 at 8:16 AM, OldRadioGuy said:

The "maths" speak for themselves. 

+Kudos for gettting the word "maths" correct!