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james70818
11-09-2010, 03:14 PM
Ok, I picked up a Vast FM01 a while back in a group buy. The other night I finally got around to making all of the connections. I built a half wave Dipole antenna for it. Frame is made of PVC and I used some SPT-1 cord. That stuff is everywhere around here. I got my size of the pole from a internet calculator. 2'-2.5" for each half based on 105.9. Hooked it up and was not that impressed with the range. About 150 yards from the antenna and I get static, 200 yards and no signal at all.

Here is what I think might be the issue but not sure. I ran the SPT-1 cable from the transmitter, with about 6' of lead then it splits to make the dipole antenna. How important is it to use coax to transmit the signal to the actual antenna portion of the setup?

Thanks, James

N7XG
11-09-2010, 03:34 PM
SPt wire is fine for the antenna, but you will need coax from the antenna to the transmitter. There are a number of designed here in the forum. Just do a search for antenna.

Dean

rjchu
11-09-2010, 03:38 PM
What Dean said. My understanding is that otherwise you're effectively making the antenna whatever length you calculated plus the 6' of run you've got going to it.

Thanks,

-joni-

IdunBenhad
11-09-2010, 03:43 PM
Hi
James, the dipole you constructed has a feed impedance of probably 200 to 300 ohms. Using the SPT-1 zip cord as your feed line IS NOT the way to go. Your FM01 has a nominal output impedance of 50 ohms. You are seriously mismatched and may cause damage to your transmitter. Also, the PVC housing may affect the antenna length. I have read that the overall length should be reduced by 19%, but I am not sure this is correct. Dirknerkle designed a dipole and put it in PVC with no apparent effects.

You should be using coax to feed the antenna. The antenna thinks the SPT-1 is actually part of the antenna, thus making it 6 feet longer than it should be. The way an antenna knows the difference between feedline and antenna is that the coax is shielded/grounded. Where the shield ends, the antenna begins.

I suggest you look at my post "FM02 Transmitter Setup" for a simple to build antenna. There are actually two parts to the post. The second set of pictures is farther down the thread. This antenna has not been tested in PVC.

Dino242
11-09-2010, 09:21 PM
I did make a PVC antenna with the length calculated at this site http://www.qsl.net/w4sat/calc.htm, using Coax from FM01 to the split (via a 3' RG cable). Mounted on the roof. And I get quite a bit more then, uhhh....200....feet....

The FM01 and antenna have been up there for 3 years, plugged in only from Thanksgiving to the New Year.

IdunBenhad
11-09-2010, 09:33 PM
Hi:
Well, more than, uhhh....200....feet.... is great. From the looks of that antenna and where it's mounted you should do about, uhhh....1/2....mile.

Nice mounting.

blickensderfer
11-09-2010, 10:04 PM
I made one along the lines of a Ramsey TM100 Tru-Match and calculated for the frequency that I needed and for testing I had it in the basement and went out driving around and I was impressed of how far it actually reached. It was 2 blocks both directions. So when I mounted it outside, it was easy mile radius.

http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=TM100

dirknerkle
11-09-2010, 11:13 PM
The Ramsey one has a lot in common with the one I put in the Wiki...;)

Andy.wpg
11-10-2010, 08:25 PM
Hi
Also, the PVC housing may affect the antenna length. I have read that the overall length should be reduced by 19%, but I am not sure this is correct. Dirknerkle designed a dipole and put it in PVC with no apparent effects.


The PVC will change the velocity factor, but it doesn't seem affect an antenna too much.

I have built a couple of twin-lead J-Pole antennas for 2 meter, one is an emergency antenna that is kept rolled up in my vehicle, the other I put in PVC. Both are exactly the same and perform equally well.

When you put an antenna in PVC, its a good idea to make sure it is centered well, that way, the velocity factor is the same no matter what direction the signal is going.

73

Andy
VE4RDO

Materdaddy
07-14-2011, 05:20 PM
The PVC will change the velocity factor, but it doesn't seem affect an antenna too much.

I have built a couple of twin-lead J-Pole antennas for 2 meter, one is an emergency antenna that is kept rolled up in my vehicle, the other I put in PVC. Both are exactly the same and perform equally well.

When you put an antenna in PVC, its a good idea to make sure it is centered well, that way, the velocity factor is the same no matter what direction the signal is going.

73

Andy
VE4RDO

So are you saying that no adjustment needs to be made when calculating antenna length that will be in PVC? I'm going to build my antenna this weekend for 87.9 MHz and am researching the length.

boyelroy11
07-14-2011, 05:28 PM
I did mine to the length given by the online calculators, and stuffed into pvc, and have a solid 1/2 mile range with 50mW and the antenna on my chimney.

I also read that a small variance from nominal length really doesn't affect the little units we use.

Bruce

dirknerkle
07-14-2011, 06:07 PM
So are you saying that no adjustment needs to be made when calculating antenna length that will be in PVC? I'm going to build my antenna this weekend for 87.9 MHz and am researching the length.

The PVC will affect the output of the antenna, but you'd have to have pretty sensitive measuring tools to notice the difference. IMO, I wouldn't worry about it.

Materdaddy
07-14-2011, 07:14 PM
Thanks for the replies! I'll post back when I get it built and tested!

ctmal
07-14-2011, 07:26 PM
As Idun said, I think the most important thing is to make sure you get 50 ohm coax to run to your antenna. I've been running mine in pvc for the last couple of years with no issues.

You can get the coax from Radio Shack.

Andy.wpg
07-14-2011, 10:42 PM
So are you saying that no adjustment needs to be made when calculating antenna length that will be in PVC? I'm going to build my antenna this weekend for 87.9 MHz and am researching the length.

TECHNICALLY, yes, adjustments should be made (I think the 19% quoted earlier is about right - I'd have to dig into the ARRL Antenna book to find out for sure), but in practice, "It don't seem to matter much".

I've never made any adjustment for PVC and everything seems to work out fine.

Good luck!

Materdaddy
07-14-2011, 11:15 PM
Thanks for confirmation. Last time I was at HRO, I almost bought the ARRL antenna book, but I was already spending a few hundred and passed for next time. If nobody is making the 19% adjustment and things are working, I question the 19% as the adjustment factor because it seems that 1/5 the length is a considerable adjustment to just be forgotten, right?

dirknerkle
07-15-2011, 08:29 AM
I think in a way you're chasing your tail here.

A lot of amateur radio "stuff" pertaining to antennas is there to make the antennas as efficient as they possibly can be, which requires perfect tuning and matching to the transmitter. This probably came about because the more efficient the antenna is, the better the transmitted signal, which becomes of paramount importance when you're trying to reach half-way around the world with a 1-watt transmitter.

There's no question that a perfectly tuned antenna will work better for even a small FM transmitter, but consider what the goal is here. The goal is to cover an area a couple hundred feet on either side of your house. You're not trying to cover all the towns in a 5-state area. IMO, for blinky-flashy use, "good enough" is just that, it's good enough. I just don't think that spending hours and potentially a lot of money to make the transmitter go an extra 50 feet is a worthwhile investment.

I chose a frequency (87.9mhz), cut the wire to match it, put the dipole inside a PVC tube, connected about 25 feet of coax to it and then to my Vastelec FM01. The antenna is mounted on a wall inside my garage and my house is covered with aluminum siding. The signal is clear up to about 1/4 mile away in my car and from that distance, you can't even see my house because the road curves. I suspect you'd get similar results if you did the same thing.

This is not to say that a *perfectly* tuned antenna wouldn't work better, it's just that there's no need for it to work better. "Good enough" is adequate.

That's my story and I'm stickin' to it...

somtng4u2c
07-15-2011, 08:46 AM
I have to say I go along with dirk

N7XG
07-15-2011, 10:05 AM
Dirk is correct. The amateurs including my self have been raised to build antennas the best way we can. You can have the best of transmitters but if your antenna sucks, well your signal also sucks. However, at the end of the day calculating antenna length for one that is put into PVC really wont have much effect on your display. I think an antenna like Dirk described on the side of your house will more than cover what you intend to transmit.

Dean, N7XG

Materdaddy
07-15-2011, 10:45 AM
I wasn't actually concerned with distance and performance of the antenna because a block or so is fine. What I was concerned with was swr and standing waves heating up the transmitter on the FM02 and prematurely failing. But, if you're all using standard calculations and forgetting about any adjustment for PVC as I've seen mentioned, I'll do the same. Thanks for the replies.

budude
07-15-2011, 11:30 AM
I have been running by EDM-LCD transmitter with the antenna it came with - an RCA jack with about 4' of stranded hook-up wire that I popped out through the front window screen. You won't find this setup in any hand book! Yet I still get crystal clear broadcasts of 1/2 block which is all I need it for. I kept saying I'm going to build one of them new-fangled dipoles but never did. I finally just got a telescoping antenna from MCM electronics to at least make it look more professional...

Unless you really need the distance (> 150 feet) I think "whatever" you put together will work perfectly fine for what we do...

IdunBenhad
07-16-2011, 08:38 AM
Hi:
As everyone has said, putting the antenna in PVC seems to have little effect. I cannot add much to the above replies to your questions, as they have all been "right on".

One thing you might want to look at is the thread "FM02 Transmitter Setup" under the forum "Radio Waves". There is a lot of information there from many people.

The antenna I described works very well and Dirks antenna is great for mounting outside. My antenna is inside a wooden shed and mounted horizontally and has a range of about 1/2 mile.

Your concern about SWR is a valid one, but the FM02/FM01 transmitters appear to handle high SWR very well, just don't let it get very high.

Good luck. Antennas are fun.

Materdaddy
07-16-2011, 11:35 AM
Good luck. Antennas are fun.

Thanks, and I agree!

I built my dipole (in PVC) last night, but had some interesting issues. The link I was using for design ideas was this one (http://www.tallahasseelights.com/tech/dipole/index.htm) which doesn't mention how to attach the copper to the chassis mount SO-259. I couldn't get the ground plate hot enough with the soldering iron (and I have a decent one) so I pulled out the torch. I got the 'ground' soldered without melting the insulating core on the connector and finished my antenna last night.

I am not happy with the antenna though. I put a 12V (unregulated) wall-wart to this LM317 regulator (http://www.sure-electronics.com/goods.php?id=65) dumping out a regulated 11.8V and fired up the FM02. In the driveway (20 feet from the antenna) there was a little bit of static on the unused frequency I chose. The interesting bit was that about a block (maybe 500 yards?) away, it was about the same reception. I don't need it to go any further than a couple houses, but I don't want static when you're right in front of my house.

I'm going to try a couple different designs with adjustments today to see if I can get a little better reception, so I'm off to buy more PVC today for a couple new antennas (and build my tune-to sign holder).

ags0000
07-16-2011, 12:04 PM
I haven't built my W3 regulated supply, or my dipole antenna, yet. However, reading everything I can find here so far, it appears many have found that it is quite easy to overmodule/overdrive the FM02. Is it possible that what you are hearing as "static" might actually be distortion? Try lowering the volume of the audio source driving the FM02 and see if that helps.

Materdaddy
07-16-2011, 01:16 PM
I haven't built my W3 regulated supply, or my dipole antenna, yet. However, reading everything I can find here so far, it appears many have found that it is quite easy to overmodule/overdrive the FM02. Is it possible that what you are hearing as "static" might actually be distortion? Try lowering the volume of the audio source driving the FM02 and see if that helps.

It didn't sound like overdrive clipping, but that doesn't mean it wasn't. Thanks for the tip, I'll try that before making my other antennae.

IdunBenhad
07-16-2011, 01:49 PM
Hi;
You didn't say if you were inputting audio into the transmitter, but if you were, it sounds like "classic" overdriving! Reduce the audio input to a very low value, or remove it entirely and listen to the signal. If you static goes away, you were overdriving.

The limiter in the FM02 does not appear to be the best in the world, so overmodulating is easy to do.

Also, be aware that it is possible that power supply noise can overdrive the transmitter, especially if the power supply is a switching type, like a PC power supply and it not adequately filtered. You can try a ferrite choke on the audio line and power supply leads to see if that helps.

Materdaddy
07-16-2011, 02:30 PM
More great tips. Thanks! I did have audio going from my phone. I turned it up most of the way, so maybe it was overdriving. Time to hook the antenna back up and re-test!

Materdaddy
07-16-2011, 04:11 PM
Looks like it was overdriving. The antenna works great. Now what am I going to do with this extra PVC? Make a 2m antenna? :D