Tram 1187 - Where do you tune it?

[Blaise]

Hi all, still just as clueless! 

 

I bought a Tram 1187 gmrs antenna for my car unit.  

On the site I bought it from, it said 'tunes easily with a hex wrench'.  So I got the antenna and installed it, and subsequently realized that I can't see a single hole or port to insert a tool into.  I looked in the packaging, and there's no instructions, so I'm kinda lost.  Anyone else use one of these, or have experience with tuning one?  The magic GOOG has utterly failed me on this one...

 

Thanks!

[maddogrecurve]

I have no clue, but I thought I would comment since you are sitting here all alone!!!!!

[maddogrecurve]

On 12/17/2021 at 11:05 AM, Blaise said:

Hi all, still just as clueless! 

 

I bought a Tram 1187 gmrs antenna for my car unit.  

On the site I bought it from, it said 'tunes easily with a hex wrench'.  So I got the antenna and installed it, and subsequently realized that I can't see a single hole or port to insert a tool into.  I looked in the packaging, and there's no instructions, so I'm kinda lost.  Anyone else use one of these, or have experience with tuning one?  The magic GOOG has utterly failed me on this one...

 

Thanks!

I did a search and you will have to cut the antenna down to tune it.  So if the SWR is high at the GMRS frequencies, 467MGz then cut 1/8" off and re test.  continue until the SWR is below 1.5, if you get to like 1.8 and you trim again and it goes up a little, quit cutting!!!!!  LOL

I have ruined many antennas by cutting too much, and some just will never tune!  

[Blaise]

What if the antenna has high swr because it's already too short?  Is there a way to tell?

[tweiss3]

1 hour ago, Blaise said:

What if the antenna has high swr because it's already too short?  Is there a way to tell?

With a nanoVNA or antenna analyzer. Open the view to 20MHz and see where the dip is. If its below 462, you can trim, if its above 467 its too short already.

[PACNWComms]

This is a common issue with many of the Tram-Browning series antennas. Many users of the three foot long antennas have ended up cutting them too short for UHF use, as well as 800 MHz in my own use cases. Then, it comes down to the skill of the person running the test equipment to check for function and SWR. I have a team member that is notorious for stating that a 5 watt CB radio puts out 35 watts, instead of 3.5 watts, using a Bird 43 wattmeter. (In that case it was NMO-27 base and whips that could be cut down to size per frequency.) It was so bad that there is now a large wood yard stick, marked with the cutting points for different frequencies in use. Sadly, I have seen many of the ones that are too short on the secondary surplus market as my employer sells its scrap, which often ends up in local auctions and later for sale at the yearly amateur radio show/convention/yard sale. These antennas are also often paired with magnet mounts where the base has corroded and the center conductor has broken. Good luck with your antenna. Measure twice and cut once.

[Blaise]

1 hour ago, tweiss3 said:

With a nanoVNA or antenna analyzer. Open the view to 20MHz and see where the dip is. If its below 462, you can trim, if its above 467 its too short already.

Oh, gotcha!  Map the SWRs to find the optimal point on the curve, and you see the center of the band.  If the slope runs off the chart high or low, you know which way you are off!

[mbrun]

Life is so much better with a VNA as compared just an SWR meter. It takes away so much of the guesswork.

Recently I was requested by a local radio enthusiast if I was familiar with a particular antenna he had been gifted. I was not familiar with it, and neither was he. In short order with the aid of the VNA I was able to ascertain what band(s) it was suitable for and just how well it was tuned for them.

I agree. If you know how to use a VNA you can tell whether the antenna has been trimmed to far or not yet enough.

One thing about trimming. Take it slow. Take off less than you think you’ll need to in-order to creep up on the final length. It may take a bit long to get there but it lessens the risk of going too far and suddenly having to purchase a new antenna. Been there, done that.


Michael
WRHS965
KE8PLM

[Blaise]

OK, so I went and re-measured frequency by frequency in a big, empty parking lot.  What I see is that my lowest SWRs are near 462.7, and my highest are near 462.5.  My DB20-G prevents transmission outside this range, but it looks to me (based on a naive linear extrapolation) like the optimal SWR will be *significantly* higher than 467.

Unexpectedly (to my own hapless self, anyway), on the lower channels, the SWRs span from 2.27 to 2.25, while on the higher channels, whose frequencies span approximately the same range, the SWRs span from 2.75 to 2.69.  Am I crazy to assume that this means the electrical characteristics change with different amounts of transmission power?

In any event, if my understanding is right (and I fully acknowledge the likelihood that it isn't!), this means that my antenna is too short.  Now, my hacker's inclination is to braze on a few inches of stainless rod and trim it back as necessary, but I feel like all my other instincts when it comes to radio equipment have been wide of the mark,so I'd really like it if you folks could give me a sanity check here!

 

 

 

[mbrun]

OK, so I went and re-measured frequency by frequency in a big, empty parking lot.  What I see is that my lowest SWRs are near 462.7, and my highest are near 462.5.  My DB20-G prevents transmission outside this range, but it looks to me (based on a naive linear extrapolation) like the optimal SWR will be *significantly* higher than 467.
Unexpectedly (to my own hapless self, anyway), on the lower channels, the SWRs span from 2.27 to 2.25, while on the higher channels, whose frequencies span approximately the same range, the SWRs span from 2.75 to 2.69.  Am I crazy to assume that this means the electrical characteristics change with different amounts of transmission power?
In any event, if my understanding is right (and I fully acknowledge the likelihood that it isn't!), this means that my antenna is too short.  Now, my hacker's inclination is to braze on a few inches of stainless rod and trim it back as necessary, but I feel like all my other instincts when it comes to radio equipment have been wide of the mark,so I'd really like it if you folks could give me a sanity check here!
 
 
 

Good evening Blaise.

The actual amount of output power from a radio does not play into SWR calculations. Yes, the ratio of forward to reflected power does play into them, but not the actual power. What is mean by this is that all else being equal, tests conducted with 1 milliwatt of power and with 50 watts of power should yield the exact same SWR results, assuming of course that the measurement instruments are designed for the levels used. If you are seeing differences between readings on the same frequency, but at different power levels, I am inclined to attribute them to the instrumentation being used. For example, some SWR meters will clearly indicate in their documentation that measurement uncertainties exist below certain level(s). Additionally, the accuracy of an instrument can vary based on the levels being measured.

From the numbers shared so far, I cannot conclude whether the antenna is too long or too short. To determine that one needs more data points. Personally, I would what to see a plot/chart of the antenna’s performance for a frequency window of perhaps 450-500MHz, perhaps even 400-550MHz. A VNA can do this.

Given that you have indicated that your radio cannot transmit outside the GMRS frequencies it then becomes impossible to plot the SWR performance over a larger frequency range which is what we really need to learn where the real dip in SWR resides.

I would encourage you to reach out to a local amateur radio club for assistance. I would expect a good percentage of their members have some form of standalone antenna analyzer, perhaps even a VNA, that reaches into the UHF spectrum. If you lived nearby I would say bring you antenna over. In minutes we would have your answer.

I wish I could be of more assistance.


Michael
WRHS965
KE8PLM