Loft mounted halfwave pipole RF saftey

[radionortheast]

theres a good one on here, dipole http://www.iol.ie/~yellowbeard/NRG%20Ki ... aerial.htm (better just screen print the dipole and the balun because website takes a long time to load) make the balun using velocity of your cable, that should stop the signal going back down into your transmitter.

the balun! it seems counter intutitive to connect the the co-ax to the inner core then the outter braid but it works..

http://www.hamuniverse.com/swr.html heres an swr chart i was looking at the other day you will see, that isn't much difference between an swr of 1.5 or 1.7 its about 95% of power reaching the aerial.

[halfwave]

theres a good one on here, dipole http://www.iol.ie/~yellowbeard/NRG%20Ki ... aerial.htm (better just screen print the dipole and the balun because website takes a long time to load) make the balun using velocity of your cable, that should stop the signal going back down into your transmitter.

the balun! it seems counter intutitive to connect the the co-ax to the inner core then the outter braid but it works..

http://www.hamuniverse.com/swr.html heres an swr chart i was looking at the other day you will see, that isn't much difference between an swr of 1.5 or 1.7 its about 95% of power reaching the aerial.

Thanks radionortheast. Only just seen your post as been very busy over the past week.

I will have a look at the links you posted.

Cheers.

Sent from my Nexus 5 using Tapatalk

[halfwave]

Hi again,

The link you gave for the dipole instructions shows a formula on how to calculate
the overall length of the dipole end to end, with the example based on 100MHz
as seen in the image below, stating the length to be 1.4025 meters:



Another site I have used to calculate the overall length of the dipole
(http://www.csgnetwork.com/antennaedcalc.html), shows a different
length of 1.426 meters, even though the frequency is the same i.e. 100MHz, see below:



Do you know why this is? and which one is the correct one to use?, as obviously I want to make
sure I make the length correct to avoid messing up the aerial. As there is about a 20mm discrepancy
between the two calculators.

[Dai Pole]

If you look at the javascript code for the "csgnetwork" calculator, you'll see that it uses a different way to calculate the length of the dipole. The equation/formula is 468/f * 1 * 0.3048 - with 1 being the whole length of the dipole (0.5 for half the length, etc.), 0.3048 the conversion of feet to metres, and 468 a constant (in feet) to be divided by the frequency.

[radionortheast]

the link I posted is closer to where you want to be, even so I find the lenghts to be a little too long, if your frequency is 100mhz it dosen’t hurt to cut each element 70 cms..

if you where using wire if it was insulated, the 70 cms would be the insulated part, you would leave a few cms to make the connection, hope that helps.

dipoleconnection.jpg

[teckniqs]

Use this for precise measurements....

http://www.csgnetwork.com/antennae3ycalc.html

[halfwave]

Thanks again for everyone's replies.

Teckniqs:

Thanks for the link. One question though, that seems to be for a yagi
type aerial. I take it it does not matter that I am going to be making
a Dipole, I guess that I just use the figure for the Calculated Total
Driven Length for the overall length of both sides.

Also, why would this calculator be more accurate than their Dipole Calculator
and the formula that Radionortheast pointed me to?

I have calculated the overall lengths using the all three calculators / calculation formula
and they vary considerably.

i.e. say for a Frequency of: 96.7MHz
I get:

1.450m (using formula 300 div 96.7 x 0.935 x 0.5)
1.475m http://www.csgnetwork.com/antennaedcalc.html
1.491m http://www.csgnetwork.com/antennae3ycalc.html

I know that getting the aerial length correct is important to match the frequency,
so I just want to make sure I am using the correct calculator.

Q1.
If the element lengths are not sized to the correct resonant length for the required frequency
it create more reflected power, right? and attaching a balun stops the reflected power getting
back to the TX. So getting the resonant length correct and the feedline balanced are important.

Therefore, Is it best to adjust the SWR / Aerial before attaching a balun, or should you attach
the balun first.

Q2.
Also when making the dipole, is it best to cut the legs a little bit longer
and then hook it up to the SWR meter, set the TX to the required frequency and then trim the
elements back until the reflected power is minimal, hoping that you don't cut off too much
and make it go higher again

[teckniqs]

Halfwave

I'd go for the http://www.csgnetwork.com/antennae3ycalc.html measurement, I use this for cutting standard dipoles as well as yagi's and it seems to work well enough for me.

I'm not saying that this will be more accurate than what else is out there but it does the job for me.
I would suggest cutting them slightly longer and taking a small piece off each time until you get the correct match just in case it wants them slightly longer than what it says on there.

[radionortheast]

make sure when your testing swr that your aerial isn’t touching anything, as that can throw off the resonance you may end up cutting each element too short..

you can use a dipole without a balun,obviously if your aerial was too short it would go back down into the transmitter.. I don’t think the balun effects the frequency too much unless i’m wrong.

[Albert H]

The balun doesn't affect the resonant frequency, just the match. I prefer the Pawsey Stub (or use a Gamma match).

[thewisepranker]

I have never tuned an aerial that ended up with dimensions matching what the calculations say they should be. Not a single one. Sometimes they're centimetres out! I'm lucky enough to own a network analyser, which I use to tune my aerials. I don't have vector functionality unfortunately as they're expensive enough in mere scalar mode, however scalar is plenty good enough. I have tuned a few aerials on a vector network analyser (VNA) in my old job and still, they came up with quite a few measurement discrepancies when compared to the maths.

The reason for this is that the maths makes lots of assumptions, like series inductance, leakage capacitance, etc. Those assumptions need to be reviewed on a case-by-case basis if you want accurate results. This is where the more expensive VNA earns its money - you can then go back and adjust the maths to get an excellent correlation.

Centimetres. How amateur.

[Albert H]

Pranker is exactly right. I started out (many years ago) with just an antenna noise bridge and an SWR bridge. I quickly found that I needed a Network Analyser, and after a good Spectrum Analyser, I spent out on one.

Antenna elements are seldom exactly as worked out mathematically, but the correct formula will give you a close approximation. Final tweaking is best carried out before going "up the block". I have a little tower at the bottom of my garden which I used to use for testing and matching aerials.

[Maximus]

I've ran all sorts of different power levels indoors. All with wildly different results.

Most of the high powered tests resulted in terrible results with high SWR readings. Even the signals didn't travel far.

Yet when I used a 5w tx with a home made end fed vertical in a bedroom facing away from where the signal should have been going, it blasted out for 5 miles, across a town and few villages away. Which sometimes I struggled to reach on 70w.

I'm still trying to work that one out to this day.


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[radionortheast]

he might be better with a gp antenna, you’ll have to see how you get on with the dipole, i’ve been messing with a dipole I made and a gp antenna, it worked alot better. I had uneven reception on the dipole, the dipole was ok, when I changed frequencies I found it didn’t get so well to the east, I shortened the coax which brought in afew extra dbs, it would get choppy, coming up strong for a secound, when I changed to a gp seem alot more steady, I couldn't believe the difference.

2.5kms coverage with the dipole, went over 3kms with the gp..I heard my signal down in a valley 8kms away..these aerials really do seem to work throw a signal to the horizon

i’m also right in thinking that the actual bottom of the radials is higher up than the bottom of a dipole would be..I was thinking of the silm jim theres chance it might not fit, the bottom will be lower down, I was thinking if he was concerned about interference etc

[Albert H]

A VHF signal always goes to the horizon (but may not be receivable when it gets there!). You extend the horizon by raising the aerial as much as possible. You also reduce absorption by buildings and trees by raising the aerial. The signal's field strength is affected by the transmitted power - the more energy you transmit, the better the signal to noise ratio at the receiver. The higher the field strength, the better.

A stereo signal has four times the bandwidth of a mono signal, so four times as much radiated power is needed to achieve the same coverage as mono.

Ideally, the field strength will be enough at the receiver to achieve "IF limiting". (Almost) all VHF receivers mix the incoming signal with a tuned local oscillator signal which is at 10.7MHz above or below the receive frequency. The difference signal - which is always 10.7MHz irrespective of the input frequency - is then greatly amplified by a single frequency "IF" amplifier and then fed to a "discriminator" circuit which recovers the audio (or multiplex signal) from the fractional variations in frequencies of the 10.7MHz signal. The final amplifier at 10.7MHz will be configured as a "limiter" - the signal will be clipped to remove all amplitude information, which doesn't matter because we just need changes in frequency. The big advantage of limiting is that it strips off interference (predominantly amplitude information), which is why a good strength VHF signal is so clean.

Receiver sensitivity figures are quoted for the onset of IF limiting - my single frequency Band IV link receivers achieve 0.35µV sensitivity for limiting. This means that completely noise-free reception can be achieved with less than half a microvolt of signal getting into the receiver. I use TV aerials at both the transmit and receive ends - with plenty of antenna gain - so the link transmitter can be just 100 - 400 mW and achieve a useful range of a few miles! Almost every building has a TV aerial, so the link Yagi aerials don't look "out of place". Also, the equipment is matched to 75Ω so that (relatively) cheap and plentiful low-loss TV aerial coax can be used with great results!

[radionortheast]

must admit I always get confused when I used to see the spec for tuners been 0.9uf , didn’t know weather a higher figure is better or a lower figure. Your maybe right about stereo, struggle to get separation without a head stomping signal, it dose make it seem more professional, on a car radio it uses stereo blend, the hiss is only slightly more noticeable. turning stereo on or off dosen't make signal stronger or weaker but changing the volume does.

car radios are to narrow to produce the deviation without compromising on selectivity, if you want to listen out of area without sounding crap, the audio has to be lower!, think thats good avice anyway if your testing out the range of trasmitter you want to beable to hear the crackles and the radio will also usually have a sign to say its in stereo st, or cd/2 disks to let you know its there

[halfwave]

Hi again,

As I have a week off work I have decided to make a new dipole.

At a frequency of 96.7Mhz the calculator it said I needed each leg to be: 745mm long,
so I cut each leg at 850mm just to give me some room to tweak.

So after hooking up my aerial in the garage to test, I know its probably not the best place
but I could not do it in the garden as otherswise the neighbours would start asking questions
I found that I got the lowest 'Reflected Power' with each radiating element at 815mm long !?!?, which Is quite
a bit off what the calculator stated.

Below are the results I have gotten for testing over a large frequency range, and as you can
see the lowest 'Reflected Power' value was at around 97Mhz

This data is based on my 1watt transmitter, with each radiating element at 815mm long.




Q1.
With the above data can someone confirm if the aerial looks to be tuned correctly for approx: 97Mhz?
as it seems to have the lowest 'Reflected Power' level and the highest Match Efficiency at this frequency.

If so, does it matter that the length of each radiating element is about 70mm longer than the value
recommended by the online calculators value? (i.e. 815mm rather than 745mm).
Or does this not matter as it's getting the best Match Efficiency at it's current length.


Q2.
Also, can anyone shed any light on why at 93Mhz I am getting more 'Forward Power' 1.25 Watts,
but only 0.94 watts at 97Mhz? As it seems to be pushing out more power, but in turn it is also reflecting
back more power therefore giving a higher SWR and lower match efficiency.


Sorry for all the questions, just want to make sure I am understanding it all correctly before I finish
it off and mount it up.

[Analyser]

Q1.
With the above data can someone confirm if the aerial looks to be tuned correctly for approx: 97Mhz?
as it seems to have the lowest 'Reflected Power' level and the highest Match Efficiency at this frequency.

Yes.

If so, does it matter that the length of each radiating element is about 70mm longer than the value
recommended by the online calculators value? (i.e. 815mm rather than 745mm).
Or does this not matter as it's getting the best Match Efficiency at it's current length.

No, but you're tuning the antenna in your garage which is not ideal by any means. I bet if you went outside and tuned it with the higest elevation you can manage, then the length would be a lot nearer the calculated figure. Buy your neighbours some dark glasses and do it outside for goodness sake!

Q2.
Also, can anyone shed any light on why at 93Mhz I am getting more 'Forward Power' 1.25 Watts,
but only 0.94 watts at 97Mhz? As it seems to be pushing out more power, but in turn it is also reflecting
back more power therefore giving a higher SWR and lower match efficiency.

The likely cause is that your transmitter is doing more power at those frequencies. The power can vary quite a bit across the band in some designs.

[halfwave]

Q1.
With the above data can someone confirm if the aerial looks to be tuned correctly for approx: 97Mhz?
as it seems to have the lowest 'Reflected Power' level and the highest Match Efficiency at this frequency.

Yes.

If so, does it matter that the length of each radiating element is about 70mm longer than the value
recommended by the online calculators value? (i.e. 815mm rather than 745mm).
Or does this not matter as it's getting the best Match Efficiency at it's current length.

No, but you're tuning the antenna in your garage which is not ideal by any means. I bet if you went outside and tuned it with the higest elevation you can manage, then the length would be a lot nearer the calculated figure. Buy your neighbours some dark glasses and do it outside for goodness sake!

Q2.
Also, can anyone shed any light on why at 93Mhz I am getting more 'Forward Power' 1.25 Watts,
but only 0.94 watts at 97Mhz? As it seems to be pushing out more power, but in turn it is also reflecting
back more power therefore giving a higher SWR and lower match efficiency.

The likely cause is that your transmitter is doing more power at those frequencies. The power can vary quite a bit across the band in some designs.

Hi Analyser thanks for answering my questions.

As the dipole is going to be mounted in
my loft is it better for me to test it in situ
so that I get the best efficiency match where it will be actually located? If not I
will try and test it outside and see what results I get.


Sent from my Nexus 5 using Tapatalk

[halfwave]

Ok after a lot of testing I got it mounted up into my loft and thought I would take
another quick SWR meter reading to make sure it was all ok... .. and to my horror
the Pfor was down at 0.5 watt, and the Pref was also at 0.5 watts also!

Something must have messed up when I installed it.. so this morning I cut all the
cables off and re fixed / soldered them and now have the aerial back to Pfor 0.85
and Pref 0.022, so all's good. I have mounted it back into my loft after sweating
buckets! as it was fricking boiling up there. Took the readings again and it's all as
it should be. So now have my old aerial and new one mounted up there. So I can
switch between the two if needs be.

Next step is to sort out a more powerful transmitter May get a 7 watt and see
how that performs.

BTW.
Would going up to 7 watts be dodgy i.e getting on OFCOM's radar? or is it still low
enough that I should be safe'ish. I know that even 1 watt is way above the legal limit,
but I don't want to push my luck, as I've been ok so far for over a year running @ 1 watt.