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Test antenna

Posted: Fri Mar 05, 2021 8:14 am
by sacentre
Hi All

I'm a complete newbie to SDR so please go easy on me! I recently bought a cheap RTL-SDR box which came with a tiny 12" vertical whip as a test antenna. I've also ordered the RTL-SDR Blog dongle and the Multipurpose Dipole Antenna kit which should be a bit better.

While, I'm waiting for the antenna kit, can anyone tell me what if any frequency range this little whip antenna would be suitable for?

I will probably get smacked by someone for not reading something somewhere but as I said, I'm completely new to the SDR ethos so please excuse my ignorance. My background (I'm a 74 year old retiree) is metal machining but my arthritic hands can no longer cope with metal shaping machines and tools so I need a new hobby to help me wait out Covid.

Thanks in advance.

Trevor

Re: Test antenna

Posted: Sat Mar 06, 2021 10:55 pm
by joe36
It would be resinate at very close to 220 MHZ fully extended and would work well at this frequency, shorter for higher frequency.
This link has a frequency calculation that will help on simple antenna design.

.csgnetwork.com/antennagenericfreqlencalc.html

Re: Test antenna

Posted: Sun Mar 07, 2021 9:52 am
by sacentre
Thanks for the reply, Joe.

It's a tiny fixed whip about 12" in height with a mag base perched on a window ledge. At the moment the ONLY signals I've been able to actually read using it are from my local FM relay (88-108Mhz) and I already have mobile phones and transistor radios for that. I tried attaching a length of wire to it and hanging it out of the window in the hope of picking up a signal or two on the HF band but again, all I seemed to get was noise. Strange, because I used to be able to pick up all sorts of stuff with my old Yaesu FRG-8800 and a random length of wire dangling out of the window.

My 2nd storey apartment room must be saturated with RF noise from all over so I'm going to have to put a lot of time into studying my rather limited antenna options. The RTL-SDR Blog dipole kit might improve things a bit. I've a ton of articles, tutorials and videos about antennas to look at so I've a lot to learn.
Thanks again.

Re: Test antenna

Posted: Tue Mar 09, 2021 4:01 am
by joe36
With the small magnetic antenna you have a good base to start. As a mono pole whip. It would work better on a metal surface such as a large pizza pan.
Better yet, If you open the antenna in the center of the magnet the coax will attach to the antenna and the shield to a washer that the magnet sticks too.
You can attach a wire to the washer, “coax shield” as a contact point allowing you to build dipole antennas by attaching wires to the antenna , “antenna terminal” and the new wire attachment point.
On the dipole antenna kit. It is a set of rabbit ears on a camera PC mount with SMA connectors and cables. It ha s several options for element lengths for 70 MHZ and higher.
If this is the frequency range you want then it is a good buy as it is a all in one kit for &) MHZ+.
If you are looking for lower frequencies then this antenna is not a good choice.
I am an old ham who started playing with the little $20 RTL sticks some years back then dropped out until this last year when I got a Hack RF1 for Christmas.
Long story short you will find the RTL sensitive and capable of receiving almost anything in its frequency range. The problem is they have no filters everything in there frequency range all at once is saturating the front end. The are very sensitive to front end desensitization when a strong out of band signal is present. An FM elimination filter will do wonders.
The best by far antenna I have found for the hf bands when using a $20 RTL is a magnetic loop.
On HF ham bands it will never be close a dedicated ham receiver.

Re: Test antenna

Posted: Fri Mar 12, 2021 7:20 am
by sacentre
Thanks for the detailed and very helpful reply, Joe. I'll try out what you suggest for the whip antenna and am looking forward to trying out the rabbit-ears kit when that arrives. Please rest assured about "long stories". I'm interested in anything experienced and knowledgeable folks like yourself care to share. I am old (74 this month) but not regrettably, a ham - only a casual but enthusiastic DXer. I haven't dabbled in the hobby for over a decade since the days when all I had was a Yaesu FRG8800 and a random long wire out the window (I've never lived anywhere where I could erect a proper antenna).

When PCs came along, I enjoyed playing around with home-brew CAT interfaces and imagined back then that more and more of what went on in a radio could probably be implemented on a PC. I never kept abreast of developments however, and only recently started to appreciate just how far things have come since - practically DC to daylight on a $10/- SDR dongle!

One of the things I especially enjoyed was decoding RTTY on the HF band using a Pakratt decoder and am very keen to explore that again. So far I've had zero success getting SDR# to talk to Fldigi via the VB cable driver and have posted questions about that problem in other forums.

I still have tons of basic stuff to learn, especially about antenna options for my very limited living environment. At the risk of stretching your patience, could I ask another question? As well as the telescopic dipole kit, I've ordered one of the cheap SA0RDT HF Mini Whip kits to experiment with. It's just the main PCB and Bias T module with SMA connectors. I need to order a 4 meter length of coax with SMA females on the ends to connect the antenna and Bias T then a short one from Bias T to SDR. My question is, is there a particular type of cable (impedance or whatever) I should look for or are they all pretty much the same? I Understand that RG-6 coax is considered optimal for antenna downleads but this is heavy and expensive and most likely won't come with SMA connectors. If the antenna shows promise, then I can think about getting a complete assembled kit with all the waterproof housings and BNC connectors installed. This will be just a lash up for now. For now, I've been enjoying using the AirSpy Server Network. It's amazing what signals can be accessed and I'm wondering if I actually need an SRD dongle in the first place.

Apologies for the long ramble and thanks again for the help.
Trevor

Re: Test antenna

Posted: Sun Mar 14, 2021 4:22 am
by joe36
I use the RG6 almost exclusively anymore. O don’t but it as it is just about every were as scrap on old satellite dishes or abandoned cable runs. I have even found roles with 400 to 600 foot on them at the good will for a couple of dollars.
You can get coaxial adapters at minimal cost from AlieExpress along with smaller diameter coax. The impendence is not critical 50 or 75 ohm is OK. The actual impedance of one of the $20 RTLs is several thousand ohms so coax at 50 or 75 is irrelevant.
That said the quality of the coax maters greatly especially above HF.
I will leave this subject a bit early tonight and will take it up at a later time.

Joe

Re: Test antenna

Posted: Mon Mar 15, 2021 2:23 am
by joe36
Followup of previous post
You had ask on a post if you need a SDR stick?
This is a question that only you can answer. If listening to web SDR is what you want.
You will have a lot of options on equipment already setup and never run out of signals.
If you want a SDR to be in your hand then, maybe?
The $20 SDRs have an A to D converter that is connected to a very wide open front end with no filtering. It is very susceptible to RF overload.
I look at The $ 20 SDR as more of a component than a complete radio. It will never compete with a dedicated radio. They are a load of fun if you live far away from close AM stations or pagers and are willing to buy or build filters.
The little SDR stick works well as a spectrum analyzer. Something that use to cost “$$ CHACHING$$I
They have a lot of potential but require some work.
You should consider getting your ham license. It is now easier than ever to pass the NO CODE test. Any local ham club can help. It usually takes a couple hours study then a test at a testing sight. The cost is almost always under $20.
Joe

Re: Test antenna

Posted: Fri Mar 19, 2021 6:44 am
by sacentre
Hi Joe
Sorry for the slow response. I've no motivation to go for a HAM license. Maybe before the internet came along I might have but then I would have had to learn morse. My only interest is receiving.

I hear what you say about the SDR dongles. For what they cost plus the free software, it's the way to go for a beginner and casual listener like me. Assuming I get a reasonable antenna set up, I might consider a mid range SDR like the AirSpy later.

My question about web SDRs maybe is a bit naive. I just wondered what I'd be missing if I can effectively piggy back on hundreds of antennas far better than anything I could set up myself. Obviously, I can't transmit and have no access to RF Gain if there is more than one user logged but I'm not sure what else. Anyway, the option's always there if my antenna efforts don't work out.
Thanks again.
Trevor

Re: Test antenna

Posted: Fri Mar 19, 2021 8:09 am
by sacentre
Joe, I hope you don't mind me coming back to you on your post of Sunday 14th about coax. Something got a bit lost in translation.

"I use the RG6 almost exclusively anymore. O don’t but it as it is just about every were .."

Are you saying you DON'T use RG-6 anymore? Both the 12" whip antenna I got with the dongle and the mini-whip kit which arrived today, came with RG-174 which I would assume is just el-cheapo stuff. Assuming I don't use RG-6 which is very heavy, what would you recommend as a better alternative? I note that 50 or 75 Ohms is not that important but the quality is.

Trevor

Re: Test antenna

Posted: Fri Mar 19, 2021 9:55 am
by joe36
I must apologies for the type O it was suppose to be a capital I.
As with tonight. I was typing in the dark. With a bit of insomnia.
YES I use and highly recommend RG6 coax.
There are some very small diameter coax good for moderate lengths at HF and below They are usually have high loss. Unless it is something special and sells for big “$”.
The RG6 is the way to go especially if you look at cost verses RF loss times distance over a desired frequency range.
The on line stuff is as good as it gets in SDR at this time and for a SWL listener who wants to use this service it is great but if you want to be the actual receiving station or you want to do signal tracking. Then an in hand unit is the only answer.
When I was a kid and would listen to a radio. It was magic. When I first got my ham license I was allowed to transmit. Now I was the magician.
Joe