Tuning a dipole (indoors)

Everything technical about radio can be discussed here, whether it's transmitting or receiving. Guides, charts, diagrams, etc. are all welcome.
Post Reply
BlackBeard
big in da game.. trust
big in da game.. trust
Posts: 63
Joined: Tue Sep 20, 2016 9:44 am

Tuning a dipole (indoors)

Post by BlackBeard » Thu Feb 28, 2019 7:35 pm

I bought this SIRIO FM DIPOLE (87-194 MHz). More information can be found here: http://www.sirioantenne.it/images/pdf/m ... 1-2011.pdf
It has a gamma match included.

I know that a great amount of RF power will be wasted, but I can't put up the aerial on a roof at the moment. Thus I want to put it up on my attic (only wood, plastic shielding and roof tiles) for 1 or 2 months and broadcast this way, until I can get it up on the roof.

The "typical SWR response" on 87 MHz should be about 1:1. I tested this with my SWR meter and got about 1:2.6 on 87.5 MHz (indoor, on my attic). The meter's power measurement should be accurate, it shows the same amount as my PCS transmitter does, so I assume that it works well. Nevertheless the transmitter heats up to 60°C in 5 minutes, although it's cooled and has a heatsink. I also checked the cable (10m Belden H-155) for short circuits several times.

Is the high SWR related to the fact that I put it up indoors (there are some wooden bars and also metal bars near the antenna, about 80cm distance) and do you recommend me to tune it this way, if I still want to put it up on my roof later?

User avatar
McDonalds
tower block dreamin
tower block dreamin
Posts: 342
Joined: Fri Aug 29, 2014 12:20 pm
Location: Essex UK
Contact:

Re: Tuning a dipole (indoors)

Post by McDonalds » Thu Feb 28, 2019 8:56 pm

I have a slimjim in my loft and gets about 5 miles if lucky but on 30 watts though its all I really need there was not much different in the garden but was to long and blown over next door but lucky don't know its for radio and nice people. but least its not windy in the loft.
the TX says my SWR is 1.5 in the loft 1.4 to 1 in the garden
My TX even makes the lights glow very dim.
but do check you not effect freeview TV.

Albert H
proppa neck!
proppa neck!
Posts: 2737
Joined: Tue Apr 05, 2016 1:23 am

Re: Tuning a dipole (indoors)

Post by Albert H » Thu Feb 28, 2019 11:31 pm

Blackbeard
ANYTHING within about 10m (three wavelengths) of your aerial will affect it. The metal bars and pipework will certainly have an adverse effect. Don't run the risk of damaging the output of your transmitter - wait until you can install the antenna properly!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

User avatar
radionortheast
tower block dreamin
tower block dreamin
Posts: 474
Joined: Wed Sep 09, 2015 1:38 pm

Re: Tuning a dipole (indoors)

Post by radionortheast » Fri Mar 01, 2019 10:19 am

when i’m messing about here, I measure the aerial when its up outside, then when indoors, I put it up diagonally across the room, so I get much the same readings, it won’t be touching the ceiling. You should beable to find the frequency its tuned to by finding the lowest reading, (you could just measure the dipole tip to tip) might of been better to get an aerial tuned for the frequency, might not be too late to change if someones coming to install it.


assuming you must be runing the pcs transmitter at maximum power, the temperature is set so the transmitter gose off at 70, its fairly rubust, if it keeps on climbing past 60, theres something wrong.
the obvious, it should scewed down to the case, even better if part of the case is thick aluminum, will work as heat sink that way, also you should have a fan. SWR of 1.5 shouldn’t be a majour problem for it, you can get away with running higher SWR with lower power.

User avatar
teckniqs
proppa neck!
proppa neck!
Posts: 3176
Joined: Thu Aug 21, 2014 11:37 am

Re: Tuning a dipole (indoors)

Post by teckniqs » Fri Mar 01, 2019 11:45 am

I wouldn't advise tuning your aerial inside. :stop

....This reminds me of a mistake I made with a PERFECTLY TUNED dipole and a 300w indoors.

A few years ago I was living in a block of flats and an annoying neighbour moved in and was continuously coming back home at stupid hours with about 15 to 20 pals during the weekday nights and blasting out his sound system.
After a this happened a couple of times I had a brain wave (or should I say brain fart) and thought I'd put my aerial underneath their sound system and see what happens.

....So put my Dipole horizontally on an 8ft pole up to my ceiling and waited for them to come back with the rig connected to a long extention lead next to my bed.

About 4am came, I heard them come in and then the music came on, so I turned the rig on and suddenly the most nasty HUMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM came blasting away from above which was music to my ears, :smoke


I then got up and went outside to listen to them because they had their windows wide open, I could hear them trying to sort it out saying "it's bad feedback, it gets worse then you walk around" etc and then about a minute later their shit music came blasting back on again and the overpowering HUMMMM had gone....

So I ran back into the front room and smoke was coming out of the BLF278.

Oops. :nodg

BlackBeard
big in da game.. trust
big in da game.. trust
Posts: 63
Joined: Tue Sep 20, 2016 9:44 am

Re: Tuning a dipole (indoors)

Post by BlackBeard » Fri Mar 01, 2019 12:41 pm

Thanks for the advice guys!

Does anyone know how accurate the transmitter's built-in SWR meter is (MAX PRO 3000+ FM Exciter)?

Albert H
proppa neck!
proppa neck!
Posts: 2737
Joined: Tue Apr 05, 2016 1:23 am

Re: Tuning a dipole (indoors)

Post by Albert H » Sat Mar 02, 2019 1:43 am

BlackBeard wrote: Fri Mar 01, 2019 12:41 pm Does anyone know how accurate the transmitter's built-in SWR meter is (MAX PRO 3000+ FM Exciter)?
It's not as good as a good quality standalone instrument, but it's good enough for relative - rather than absolute - indication.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

User avatar
trancetechnic
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 34
Joined: Mon Jul 23, 2018 3:51 pm
Location: Glasgow
Contact:

Re: Tuning a dipole (indoors)

Post by trancetechnic » Sat Mar 02, 2019 2:19 pm

Hi ! Anyone know what this antenna is called ?
:tup

User avatar
teckniqs
proppa neck!
proppa neck!
Posts: 3176
Joined: Thu Aug 21, 2014 11:37 am

Re: Tuning a dipole (indoors)

Post by teckniqs » Sat Mar 02, 2019 2:38 pm

Circularly Polarized Dipole

User avatar
trancetechnic
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 34
Joined: Mon Jul 23, 2018 3:51 pm
Location: Glasgow
Contact:

Re: Tuning a dipole (indoors)

Post by trancetechnic » Sat Mar 02, 2019 3:08 pm

Many thanks.
:tup

Albert H
proppa neck!
proppa neck!
Posts: 2737
Joined: Tue Apr 05, 2016 1:23 am

Re: Tuning a dipole (indoors)

Post by Albert H » Sun Mar 03, 2019 5:55 pm

trancetechnic wrote: Sat Mar 02, 2019 2:19 pm Hi ! Anyone know what this antenna is called ?
It's a slightly radiating dummy load! It's actually a circularly polarised dipole, but they really radiate less energy than is put into them - the loss varies from 2 - 3 dBd (that's dB against an "ordinary" dipole) - according to manufacturer. You can stack them - two - properly co-phased - give (roughly) the same field strength as a vertical dipole, but with one (supposed) advantage:

Circular polarisation is supposed to penetrate between and into buildings better than linear vertical polarisation, and to be less prone to multipath distortion problems. It was supposed to enhance reception in moving vehicles and into receivers with randomly polarised receiving aerials (like you wouldn't pull the telescopic aerial on your radio straight up to start with). Circular was a late 70s and early 80s fad in the broadcast world.

I've used lots of both kinds of aerial, and the ordinary vertical dipole - especially when stacked - gives far superior results. Circular does bugger all for the listener, except give them a weaker signal.

The only real 70's enhancement to FM broadcasting was FMX - a system developed in the 'States and Japan that added an extra "S" subcarrier in quadrature to the usual one. People with mono receivers would get the "M" signal (left and right combined) as usual, stereo receivers would get their "M", pilot and "S" components in the usual way giving stereo reception, and those with FMX receivers would get the whole thing including "S" and "S'" signals.

FMX doesn't suffer from the "picket-fencing" effects of ordinary FM stereo, and allows better reception both when mobile and with fixed receivers. It was a great system, and a lot of US stations installed the coders (just slightly more expensive than an ordinary FM Stereo coder). Several receiver manufacturers offered receivers for the system, and Ford cars were fitted with FMX receivers (as an option) for a couple of years. My little station in the Hollywood Hills was probably the only pirate to transmit FMX!

Sadly, the arrival of IBOC (don't get me started on that waste of space) and satellite radio killed off the fledgling FMX system.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

Matt
big in da game.. trust
big in da game.. trust
Posts: 86
Joined: Sat May 06, 2017 9:41 am

Re: Tuning a dipole (indoors)

Post by Matt » Sun Mar 03, 2019 9:00 pm

Seeing that made me a little nostalgic! I know nothing about the technical side of transmitting (so I've no idea why I find this part of the forum so engaging) but I had the chance to go and have a look at the aerial of a legal station I worked for in the early to mid 00s. I'd gone to the transmitter to replace the CD for silence detection at the transmitter and the security guy asked if I wanted to go outside onto the roof. As well as a fairly unique look at the city after dark, he pointed out the aerial and it was one of those.

I've seen one or two other antennas in my time at various broadcasters of differing legal status, but never one of those. Always wondered quite what it was!

Albert H
proppa neck!
proppa neck!
Posts: 2737
Joined: Tue Apr 05, 2016 1:23 am

Re: Tuning a dipole (indoors)

Post by Albert H » Mon Mar 04, 2019 6:37 pm

As I explained - they were supposed to improve reception in built-up areas, but all the tests that I did with various polarisation schemes usually showed that vertical won out in most circumstances. Several manufacturers made the CP aerials, and charged big bucks for them.

I remember demonstrating the difference in field strength to the CEO of a station in Spain with the three aerial options he could have. We showed him a stacked pair of dipoles against a 4-stack of Circularly Polarised and against a twin-slot colinear. All three antennas were about the same height and were mounted to three posts on the top of a building. The Twin Slot Home-brew Colinear made the others look defective: The prices of the three antenna increased in inverse proportion to their efficiency!

Remember - the cheapest amplifier is a good antenna. Stephen and I proved time and again that 40 Watts into a good aerial will beat 200 Watts into a dipole.

If you want to build your own aerial, look at "Harry's Homebrew Homepage" and look at the colinear on there. It's easy to build and match and will give you almost 6dB of gain in the directions you want! http://213.114.131.21/antennas/6dbvhf0.htm shows you how to build it.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

BlackBeard
big in da game.. trust
big in da game.. trust
Posts: 63
Joined: Tue Sep 20, 2016 9:44 am

Re: Tuning a dipole (indoors)

Post by BlackBeard » Mon Mar 18, 2019 4:26 pm

So I finally managed to tune the antenna to 1.2:1 after someone in the PCS-forum told me a little secret about the gamma match (there's a little pipe that can be pulled out). Made a little test run today - 6 hours of UK garage ;)

Post Reply