Measuring losses

[Blaise]

After a fair deal of work, I've tuned my Tram-Browning through-glass antenna to an SWR of about 1.2 at 465 MHz.  So now my question is, what about other losses?  SWR, as I understand it, is measuring the transmitted energy that bounces back to the transmitter, but that can't be the only potential power-loss mode for transmission, right?  Resistances must also play a part, as well as some of the bizarre LCR effects I vaguely remember from my circuit design class several decades ago, right?  I'm assuming the inductive coupling in the through-glass transducer must produce a loss too, given the distance travelled through a dielectric...

So how do you measure these losses?  Obviously SWR is most important, what with how it can burn up your radio, but I'd also like to know how to measure the real power making it into the actual transmission...

[marcspaz]

 

 

You need an RF Field Strength Meter that can read the magnetic and electric fields at X # of meters from the antenna and compare the output to another antenna install on the vehicle  with a direct-coupled antenna.  They are about $400 for a meter worth buying.

 

You also need an antenna and coax cable analyzer.  They can measure cable and connection losses.  Good one will even tell you exactly how electrically and mechanically long a cable is.  Expect to spend about $550-$600 for a meter worth buying.

 

EDIT:  Forgot to add, there is likely such small loss anywhere in that path (since its in a car and the runs are so short), that there is likely no room for any measurable improvement.  Even if you make a measurable improvement, there will likely be no practical improvement that will be distinguishable by the operators on either end.

[SteveShannon]

1 minute ago, Blaise said:

After a fair deal of work, I've tuned my Tram-Browning through-glass antenna to an SWR of about 1.2 at 465 MHz.  So now my question is, what about other losses?  SWR, as I understand it, is measuring the transmitted energy that bounces back to the transmitter, but that can't be the only potential power-loss mode for transmission, right?  Resistances must also play a part, as well as some of the bizarre LCR effects I vaguely remember from my circuit design class several decades ago, right?  I'm assuming the inductive coupling in the through-glass transducer must produce a loss too, given the distance travelled through a dielectric...

So how do you measure these losses?  Obviously SWR is most important, what with how it can burn up your radio, but I'd also like to know how to measure the real power making it into the actual transmission...

If you put a watt meter at the antenna connector on the radio and compare the power there to what a watt meter measures when connected right at the antenna you should be able to measure your losses. 

[Blaise]

3 minutes ago, Sshannon said:

If you put a watt meter at the antenna connector on the radio and compare the power there to what a watt meter measures when connected right at the antenna you should be able to measure your losses. 

Here's the thing, though, the connector for the antenna is on the transducer, which is inside the glass, so measuring at that end of the cable would ignore what I assume will be non-trivial losses due to a) the transducer's internal circuitry, and b) the through-glass connection itself.  Is there a way to capture that data too?

[SteveShannon]

4 minutes ago, Blaise said:

Here's the thing, though, the connector for the antenna is on the transducer, which is inside the glass, so measuring at that end of the cable would ignore what I assume will be non-trivial losses due to a) the transducer's internal circuitry, and b) the through-glass connection itself.  Is there a way to capture that data too?

There’s probably a way with a field strength meter, but I that’s beyond my experience. 
Is there no way to remove the antenna element from the other half of the coupler?

I doubt that the losses are great there anyway. There’s no heating, right?

[kidphc]

There’s probably a way with a field strength meter, but I that’s beyond my experience. 
Is there no way to remove the antenna element from the other half of the coupler?
I doubt that the losses are great there anyway. There’s no heating, right?

Actually, a field strength meter with be awesome. You can set up multiple test and remove variables to get the answer. Knowing the gain of the antenna, coaxial losses etc. You could derive a formula.

In conjuction with the swr meter, you could get some more of the values.

Unfortunately, the swr meter is deriving power and swr readings using some calculations. But really are only good to see if you have a resistance mismatch for the tune circuit ( radio, antenna, feedline). It however is better then nothing.

Glass also is unfotunately, has great thermal and magnetic insulation properties (emf and ir for instance). Magnetic forces can make its way through, they have to be strong enough though.

Easiest way to calculate the losses is to ask the manufacturer. Hopefully, they will be forthcoming.


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[Blaise]

30 minutes ago, marcspaz said:

You need an RF Field Strength Meter that can read the magnetic and electric fields at X # of meters from the antenna and compare the output to another antenna install on the vehicle  with a direct-coupled antenna.  They are about $400 for a meter worth buying.

You also need an antenna and coax cable analyzer.  They can measure cable and connection losses.  Good one will even tell you exactly how electrically and mechanically long a cable is.  Expect to spend about $550-$600 for a meter worth buying.

EDIT:  Forgot to add, there is likely such small loss anywhere in that path (since its in a car and the runs are so short), that there is likely no room for any measurable improvement.  Even if you make a measurable improvement, there will likely be no practical improvement that will be distinguishable by the operators on either end.

Woof, I was kind of expecting it to be that complicated/expensive.  I certainly hope you're right about the negligible losses!

 

22 minutes ago, kidphc said:

Easiest way to calculate the losses is to ask the manufacturer. Hopefully, they will be forthcoming.

Sadly, as far as I can tell, there is no way to determine the identity of the manufacturer for Tram-Browning antennas, much less contact them.  That's actually what got me interested in going down this path.  Most comments I've gotten about these antennae are highly negative, and in the vein of, "You're losing 50% of your power just getting through that glass.", "That 25% tint on your window is eating up your transmission power", "Incoming signals will be so attenuated you'll lose half your range", etc...

I opted for through-glass because of the low cost and extreme ease of installation, and they *seem* to work quite well, if you can get them tuned (which does seem to be quite a challenge), but I'm just wondering about the veracity of those claims...

 

[kidphc]

Sadly, as far as I can tell, there is no way to determine the identity of the manufacturer for Tram-Browning antennas, much less contact them.  That's actually what got me interested in going down this path.  Most comments I've gotten about these antennae are highly negative, and in the vein of, "You're losing 50% of your power just getting through that glass.", "That 25% tint on your window is eating up your transmission power", "Incoming signals will be so attenuated you'll lose half your range", etc...

I opted for through-glass because of the low cost and extreme ease of installation, and they *seem* to work quite well, if you can get them tuned (which does seem to be quite a challenge), but I'm just wondering about the veracity of those claims...
 

They work. Lot of hams don't like them. Why you are going to waste energy transferring between the inductive coils. They don't work well above 800mhz, not a problem for gmrs.

If it works be happy.

They only way to know is to use it and get reports. You really won't know the differences till you throw a mag mount on the hood or roof. Then you'll get a better idea of the differences.

I went from a through the roof nmo to a hood hinge mount. Were there noticeable differences? Yes. More picket fencing a bit loss of range, (some repeaters I can't even hit in certain locations). Was trade off acceptable to me? Jury is still out.

Don't sweat it too much.

I looked a the Larsen glass mount to get idea of losses, even they don't publish them. They tend to publish almost all their spec some where or another.

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[BoxCar]

You can pick up a decent miniature VNA (Vector Network Analyzer) for less than $100 that will tell you many things about your installation. VNAs can measure the SWR, tell you what the resonant frequency of your antenna is and show you what must be done to get the antenna at its lowest losses at the frequencies you use.

[SteveShannon]

There are patents for new glass couplers that cite the high losses of capacitive coupled devices. Apparently there are at least four different ways of through glass coupling. 
https://patents.google.com/patent/US6661386B1/en
You also must consider that an antenna connected that way doesn’t really have a ground plane (I presume) so it’s starting with a disadvantage anyway. 
But as they say, “Any antenna is better than none.”

Use it and see if it works well enough. 

Edited June 1, 2022 by Sshannon
Added patent reference

[kidphc]

Never asked if it was a soft top or hard top. Another option would of been to mount the antenna inside. Definetly, not ideal, but the fiberglass and soft top would of been some what transparent to the rf.

Hopefully, (kinda expect) it's not mounted to moving glass. Which is a no no.

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[Blaise]

18 hours ago, BoxCar said:

You can pick up a decent miniature VNA (Vector Network Analyzer) for less than $100 that will tell you many things about your installation. VNAs can measure the SWR, tell you what the resonant frequency of your antenna is and show you what must be done to get the antenna at its lowest losses at the frequencies you use.

Yes, I've had my NanoVNA for about a month now, and it's how I finally got this dumb thing tuned as well as I have.  It saved a whole lot of time being able to see the whole curve!   With the SWR meter, all I could do was map the local slope, so I kept getting caught in local minima...

[Blaise]

1 hour ago, Sshannon said:

You also must consider that an antenna connected that way doesn’t really have a ground plane (I presume) so it’s starting with a disadvantage anyway. 

It has a strange secondary antenna that extends out inside the glass which I assume is the equivalent of a "ground plane" for it, but I still know almost nothing about antenna physics, so I could be wrong!

[Blaise]

1 hour ago, kidphc said:

Hopefully, (kinda expect) it's not mounted to moving glass. Which is a no no.

LOL  No, it's mounted to the vertical fixed window at the back of my truck's cab.  (I'm new, not dumb!) (I hope...)