SWR meter - Do it yourself project

[Pollux]

Hi,

I've done a forward voltage test with RG402 on Tx->coupler and RG402 on coupler->RF-detector.

always with the same 200mW chinese Tx (5.645 to 5.945 GHz) and a homemade dummy load, I get Vf= 2350 mV to 3150 mV, depending on the channel (lower value on lower frequency), and Vr= 65 mV to 135 mV. The higher Vf correspond to higher Vr... It gives a VSWR of 1.06 to 1.09... not so bad for a homemade dummy load...
with RG316 on Tx and detector, and the same setup (same Tx, same power supply, same load, same detector diode, resistor and capacitors, all disposed exactly the same way), I get Vf=150 mV to 860 mV, and Vr= 2 to 10 mv, giving a VSWR of 1.02 to 1.03...

my conclusions:

1/ RG316 is very lossy at 5.8 GHz. With RG402, I get more constant Vf over the frequency range of the Tx, and the mean value of Vf is 5x higher... !!!

2/ low Vf means low Vr, barely undetected, giving false and optimistic SWR readings...

3/ my homemade dummy load is not so bad... but is not perfect. Also, I can't know for sure that it's my dummy load which is not perfect or if the coupler has a swr of 1.05... the datasheet of the coupler specified "vswr<1.20 from 4 to 8 GHz"... ... (idea: I'll make a very good cloverleaf and I'll see if I can get a lower Vr than with my dummy load... ;-) ) [EDIT_1] for now, this idea failed... 1.22 is my best SWR with CL... still have progress to do... [EDIT_1]
[EDIT_2] I did it !!! swr = 1.00032 (@5.885 GHz, Vf=1850 mV, Vr=0.3 mV) with a 5 turn 5.8 GHz helix antenna... my dummy load isn't as good as I thought [EDIT_2]

4/ RG402 rocks !!! It's expensive, hard to cut /work with, but it's very very good. I'm impatiently waiting my RG405 pigtails...

5/ I can't tell for 2.4 GHz (I will soon have a coupler for this band too), but for 5.8 GHz, I will not use RG316 again.

[Jack2010]

So, first of all I don’t know if I should post it here or there: http://fpvlab.com/forums/showthread....-s-Guide-VIDEO, but decided to start within this thread.

I am building my first FPV gear that will be used in the MWC quadro. Read a lot about Cloverleaf (CL) and Skew-Planar Wheel (SPW) antennas promoted by many just to mention IBC among them and decided to build my own set for 1280MHz as they are so cool.

Did the first pair some time ago without putting too much effort into it. Thought that this will be like a training before I will make the right one. So, in the mean time got also this marvelous tool for measuring VSWR from Michael. Did some measurements (FOX800) and got some results:



Not that bad as for the first build and the one I was not so orthodox on precision.

Now, here is my problem - just last night finished the build of new CL (SPW is still under construction) but this time I have been very accurate, benefit previous experience putting a lot of time and effort in order to make it right... and the outcome is almost the same!



I have to say that I was truly unhappy at about 2 am tonight especially if you compare amount of time and effort put in the second build vs. the first one.


Now, is there anyone can help me understand why am I having such high values on Vr as I believe that is the source of my problem. What are the key factors impacting Vr while building CL and/or SPW?

[Pollux]

Hi Jack2010,

Could you post some pictures of your antennas and your SWR meter?
the problem may come from both systems.
your swr meter could easily have a native SWR of 1.4...
or your antenna could be wrong in geometry
or your connections between plugs and coax could be mismatched...

there's a lot of things that can go wrong.

for the moment, my best antennas gives me VSWR = 1.22... and I am very careful and not far from being a soldering master...

try to get your hand close to the lobes while measuring Vr (@1280 MHz). if Vr increases, your antenna is too long.
if Vr decrease, your antenna is too short.
then, try to get your finger close to the feed point of your antenna. if Vr increases, your antenna is too short
if Vr decrease, your antenna is too long.

if, with these tests, Vr increase every time, it means that your problem is not in the length of your antenna wires.
it may be a geometry problem, or a mismatch in your coax routing, or your SWR meter has a 1.5 SWR...
if your coupler is a homemade one, you could try this trick:http://www.arrad38.fr/bidouilles/ros...metre_23cm.htm
sorry, it's in french... the trick is showed in this picture: http://www.arrad38.fr/bidouilles/ros...de_reglage.jpg

just remember one thing: swr=1.6 is already a 94% efficiency of your antenna... you're chasing 5%.

[Kondor-FPV]

Here are a couple pictures of a 5.905Mhz (ch6 HK tx) pinwheel antenna.

[Jack2010]

Ok, so I cannot show you too much with respect to the CL as it is finished, but I can promote my SPW as it is under construction. However the CL was made in the very same manner and components are exactly the same so this should give you same good reference. There it is (without coax yet):









And this is CL we are talking about:



And the old creations:



And Michael (Mictronics) SWR meter:



Did the test you ask for and:
- Vr is going down by ca. 0.25V when I close my hands to the antenna like I am creating the umbrella surrounding it
- Vr is going up by ca. 0.25V when I close tip of my finger to the feed point been perpendicular to the coax cable from the side of the antenna
- Vr is going down by ca. 0.25V if I close my finger to the top of antenna/ feed point in the way that my finger is in line with coax and my hand is above the antenna feed point


Assuming my antenna is to short, what exactly do you mean by that? The entire lob is to short or particular section is, or else?

[Pollux]

Hi,

these antennas look great, I don't see any geometry problem... maybe your SPW is a bit too flat, but I'm not sure.

what I mean by "too short" is that the wire you used for the leafs of your antenna may be a bit short for this frequency.
you can try to increase the length of the feedpoint, by 1 or 1.5 mm, just to see if it's better.
ooops, you hot-glued it... you can't do this anymore...

but I'm not sure that your antenna is too short...
if it was too short, Vr should be going UP when you close your finger to the feedpoint in line with the coax...
I have had the same problem with a 5.8 GHz CL.
The "surrounding hand" test showed a too short element (Vr going down), and the "finger tip" test showed a too long element (Vr going down too)...
In my case, the SMA pigtail was the problem... The insulation was cricked... but that was on 5.8 GHz and I don't think that 1.2 GHz is that sensitive.

you can do two tests:

1/ increase your feed length (it will give you a "longer" antenna) and do the same two tests ("surrounding hand" and "finger tip")... that means removing the hot glue, without damaging your antenna... not an easy trick.

2/ if you have a dummy load, do a SWR test with the load instead af the antenna. This will give you the SWR of your coupler.
if you don't have a dummy load, you can do one with this method: http://pe2er.nl/wifiswr/ (at the end of the page)
It's better to use non inductive resistors, but for 1.2 GHz, I don't think it's critical.

[coolest]

i would like to buy 1.2Ghz DIY SWR meter, but i need 23 more post

[roberto]

built a second SPW following Sircana´s tutorial and some small mods for placement on a wing. I´m a happy camper





special thanks to: IBCrazy, Sircana and Mictronics.

[fredz]

Looks good

[Nikotine]

Hi guys, I need your advice.
I built this meter using SMD parts. I used an HSMS-286K, a 1K resistor and a 0.01 µF capacitor.
I actually had to restart 2 times because the pins on the diode broke, it's a pain in the ass to solder this!






The trouble is that Vf keeps going down, it doesn't really settle.
Vr is stable after a few seconds.
How can this be?
Should I rebuilt it with normal parts instead of SMD?

These are the values I get:
Cloverleaf built for 1320 MHz:

Cloverleaf			
MHz	Vf	Vr	p	SWR
1240	1,36	0,408	0,3	1,857142857
1280	1,404	0,281	0,20014245	1,500445236
1320	1,352	0,0821	0,060724852	1,12930152 <---
1360	1,095	0,3596	0,328401826	1,977971172

This is a cloverleaf built for 1280 MHz:

Cloverleaf				
MHz	Vf	Vr	p	SWR
1240	1,036	0,3457	0,333687259	2,00159351
1280	1,477	0,0406	0,027488152	1,056530214
1320	1,347	0,0165	0,012249443	1,024802706 <----
1360	1,101	0,1422	0,129155313	1,296620776

Apparently I didn't do well there...

And this is a SPW for 1280 MHz:

Skew-Planar Wheel 			
MHz	Vf	Vr	p	SWR
1240	1,4743	0,109	0,073933392	1,159671867
1280	1,3855	0,077	0,055575604	1,117692014 <---
1320	1,263	0,107	0,084718923	1,185121107
1360	1,106	0,2153	0,194665461	1,483439991

Do these values make any sense?