Making the perfect matched antenna

[XXL]

Iv often heard people talk about the cheapest way to get more power is to upgrade the antenna, and I’ve seen it happen. I had a rig builder who built me a perfectly matched jpole once, and the signal went well over 10x the distance compared to a dipole (without balun).

Unfortunately the builder doesn't make them anymore, but I’m trying to work out how they did it. It had a loop of coax on it which had to be a certain velocity and length to perfectly match along with the correct element lengths.

My question is what does the loop of coax do, and do I need some sort of antenna analyser to do this or can I get very close using some sort of home brew device ?

[s2000]

It's supposed to stop the feedline radiating (a common problem with jpoles). What I believe the coil does is make a simple rf choke which stops the rf travelling down the coax (effecting the radiation pattern). You can usually tell if your jpole is radiating by touching the coax under the antenna, if the swr goes up it means it is likely radiating. Also clipping the cable down the mast can cause a problem (without coil), the swr can go through the roof lol

I did have all the above problems with one of those Ebay jpoles, even a coil wouldn't stop the feedline radiating. In the end I had to buy a large ferrite ring and wrap a number of turns of the coax (not easy when its rg213!). The feedpoint also had to be changed to a much lower position. But eventually I got it spot on, no radiating and swr was 1.02:1 on the rig analyser.

It can be a lot of hard work tbh, the next antenna I am going to try and build is Harrys one, I'm hopiing they're not a problematic as jpoles!

[XXL]

Those rig experts are so expensive for what I need it for. Is there a cheap way to get a close match ? I can switch on a transmitter and play about with it until the swr is better but that’s not exactly perfect.

[teckniqs]

Your dipole wasn't working properly if your J-pole went out 10 times the distance.

[Bton-FM]

You could just get an swr meter if you want to match it.

[Ignite]

Here's an interesting video about antenna tuning. The bit you want to watch starts at about 20 mins in. http://thisweekinradiotech.com/twirt-ho ... opolo.html

[teckniqs]

You could just get an swr meter if you want to match it.

Not good enough, a decent antenna analyser would be a lot more suitable for tuning these types of aerials.

[Bton-FM]

You could just get an swr meter if you want to match it.

Not good enough, a decent antenna analyser would be a lot more suitable for tuning these types of aerials.

What aerials would you say are suitable for building with just an swr meter?

[teckniqs]

Not this one

[Tec1 87.5fm London]

J pole

[teckniqs]

J pole

So why do you build yours with an MFJ antenna analyser, then?

...It's all well and good just using a SWR meter to tune your J-pole, but if it's matching a good SWR at say 100ohms you could damage your transmitter and the aerial won't work properly.

[Albert H]

You can also find a "conjugate" match - looks like 50Ω resistive, but is mostly virtual. Bizarre things can happen!

The Made From Junk 259 used to be the standard "ham" antenna analyser, but nowadays there are cheaper and better ones coming out of China. Don't buy the cheapest one you can find - go for the second cheapest - they have the better chipset in them!

[Albert H]

Incidentally - I have a random bit of wire (it measures about 0.65 of a wavelength at 87.7 MHz) hanging in the middle of my workshop at the moment, with a little 800mW exciter board dangling from it. It's going about 4km into town in one direction, and about 3 km to the beach in another! The match is pretty much 1 : 1 despite it being inside a brick-built building just 9m ASL. It gets out better than most London pirates!

The little rig is being fed 12V DC from a gel cell, and the modulation is actually MPX stereo. The plan was to try out the little RDS board I've been messing around with, but it's a nice day here, so it's off to the beach with a portable radio to throw sticks for my dog while it's still light!

[XXL]

What about matching the output to A 50ohm dummy load and then change to an antenna to get the lowest swr possible?

Ignoring other factors at transmission site btw.