Antenna Considerations / Do's and Don'ts
Hi All,
I am considering my antenna options. While my cheap magnetic loop does a good job, there are many signals such as DRM that need a bit more gain - otherwise they simply don't work.
This seems quite reasonable for the price, an end fed long wire with a 1:49 balun. And covers 1-30Mhz.
What other considerations should I be thinking about before leaping? Attenuators, Filters, other options.
I have a reasonable size section (good old kiwi 1/4 acre). I also have access to a good sturdy pole, as we are a WISP here in the Hawkes Bay, and have some nice poles that can be mounted on a pad.
Anyway, looking for a little guidance to an age-old question. However, it does help when there is so much information and advice on the internet but no real clarity on what to use.
Comments
Unbalanced antennas are prone to noise pickup on the feed line.
Use a balanced antenna if you can, as this will provide better results.
Concentrate on the Signal to Noise Ratio rather than absolute signal strength.
A good transmit antenna is not necessarily a good receive antenna. Antenna efficiency is important on transmit, but much less so when only used for receive.
Site an antenna for the lowest noise level rather than the strongest signals.
With receive systems, it is the Signal to Noise Ratio that matters. As long as the antenna gain / sensitivity is sufficient to raise the receiver noise floor by 6dB relative to a screened 50 ohm load, then you can't improve the antenna performance any further, other than by increasing the directivity.
Directivity increases the gain in the required direction, but more importantly reduces gain in unwanted directions, including noise sources. If you have much more than 6dB increase in noise floor, you are eating away at your receiver's dynamic range, and this is especially important with wideband receivers such as the KiWi.
Owen Duffy (one of my heroes) introduced the concept of Signal to Noise Degradation, and he has many posts on the subject.
https://owenduffy.net/blog/?p=30632
ITU noise curves and required receiver noise figure. For reference, the KiWi NF is around 14dB, which is adequate at 30MHz.
https://owenduffy.net/blog/?p=15700
As you can see, a very high noise figure receiver is adequate at 1MHz, which is why electrically short active antennas can still work well at such low frequencies.
Regards,
Martin
Thanks Martin.
I am primality looking at an RX antenna for my KiWi.
Low noise would be great.
Techminds use an end feed balanced antenna when comparing against other ones for review, and he seems to have a really low noise floor and good SNR.
This is what my poor kiwi is reporting.
0-30 MHZ SDR | NAPIER, NEW ZEALAND
http://21996.proxy.kiwisdr.com:8073
KiwiSDR 2 v1.699 📡 GPS 📻 DRM (0/4 users, SNR 7/3 dB) where others in the public list are reporting over 40!
An End Fed wire antenna is by it's nature not balanced, and they have a reputation as tending to be noise magnets. A wire dipole, loop or active loop are better options.
With your KiWi a lack of signal level seems to be the main issue. Even if there are few signals I would expect to be seeing more noise.
Is the loop antenna (I assume an MLA-30) powered correctly ?
If it is an ML-30 have you experimented with the two gain controls (one in the bias tee, the other in the loop amplifier) ?
Regards,
Martin
I watched one of Techmind's video's relating to the End feed antenna.
https://youtu.be/VYt3-jkBfaQ?feature=shared
Some things of note.
He is on the edge of rural farmland, so the noise floor will be lower than if he was in the middle of at typical UK housing estate, it is also likely that his utility services will be buried rather than being carried by overhead wires, which will further improve the noise floor.
He has installed the antenna in the best way possible, with the feed point located way from the property, and he has also installed a choking balun and separate RF ground, although this could be further improved. Ideally, he should also have incorporated some form of galvanic isolation, in order to reduce the risk of ground loops, or possible issues with UK wiring regulations and mains safety.
Regards,
Martin
Ah of course an end fed is unbalanced...
Yes, its an ML-30(+) however I havent messed with the gain controls.
Ill have a look tonight. ;-)
Thanks
Sean
I'm considering some kind of multiplexer and have two ML-30's so one is north-south and one is east-west orientation.
If you simply combine the two antenna's, you will change the direction of the bi-directional pattern, but you will not achieve an omni-directional response, unless you combine them using a 90 degree hybrid to produce circular polarisation.
Better to switch between the two antennas, or build a switched combiner, Loop 1, Loop 2, Loop 1 & 2, Loop 1 with Loop 2 anti-phase, which would allow you to steer the nulls to a certain extent.
Regards,
Martin
I wound up the gain on both the antenna preamp and the Bias Tee. The Bias Tee is quite sensitive, and ramps up the noise floor, so its a balancing act. I'll keep tinkering, however I can now decode DRM signals I couldnt before, due to lack of signal strength.
The gain control on the bias tee is a passive one, so it's probably best to set that for minimum attenuation and maximum signal levels anyway.
Then adjust the gain control on the loop amplifier board.
There is no point adding more gain than necessary in the loop amplifier, only to counteract it by applying attenuation later in the signal chain.
Ideally, you should aim for approximately 6dB increase in the KiWi noise floor at frequencies around 20 to 30MHz, when the antenna is powered.
Any more than this, and you will not improve the Signal To Noise Ratio, but you will be eating away at the receiver's dynamic range, and making it more prone to overload on strong signals.
A 6dB increase in noise floor, when the antenna is connected, is the "sweet spot" to aim for.
Regards,
Martin
I found that the gain control on the bias tee had the most overall affect, however it did increase the noise.
I'm not on SNR 22/16 dB which is a big step up from under 10.
The control on the antenna had the least affect. I may have wound up the bias tee one a bit much, however, I took it to the point just below where the waterfall was green with noise and the signals were a few db higher.
I'll have a tinker with it over the weekend, and watch some YouTube videos with others experiments.
When using an MLA-30, I don't think you can have too much gain :-)
Auto waterfall settings can have you chasing your tail. So I tend to set the KiWi waterfall & Spectrum display to manual, zoom fully out to see the whole spectrum, then set the maximum waterfall level to -20dB and the minimum to -110dB. This gives a good baseline for comparison with other KiWi's when the same settings are used on them too.
Don't worry about the noise increasing, you should see some, especially in an urban environment.
Just aim for a 6dB increase in the noise floor on the upper HF bands 20-30MHz, when the MLA-30 is powered up.
As I said before, turn the bias tee control to maximum gain, and then just use the amplifier gain control to achieve the best overall performance.
Regards,
Martin
Ill give it another tweak tonight :-)
Well, interestingly, the pot on the amplifier itself did nothing noticeable with the signal strength or noise, however the pot on the Bias Tee has the most dramatic effect.
I assume the Bias Tee pot is LNA gain and the pot on the amplifier is IF gain.
I have the LNA gain set to just shy of max and the IF turned about 80% to max.
Ill see how that goes tonight once the sun has set.