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HF Antenna length for Reception SDR (1 reply and 1 comment)

Phil M7PHX
7 months ago
Phil M7PHX 7 months ago

Hello. I've recently strung up a 1/4 inverted V dipole for 20m (2x 5m lengths of wire). I don't have an HF transceiver, just an SDR dongle.

How critical is the length of the antenna wire for reception? I cut my wires are bit long 5.4m so that I could trim them, but I don't know how critical it is for just receiving?

Also without an Antenna tuner, SWR meter etc. Is there a way of checking with software when using an SDR dongle? 

How much does the height of the antenna effect reception for HF specifically 20m? 

one last question, is there a usual bandwidth for transmitting on 20m?

thanks in advance


Peter M0PWX (2E0PWX)
7 months ago

Hi Phil

great to see another M7 getting on the air

for RX the tuning of the antenna is not critical, the higher you can get it the better (as with TX) 

if you plan to work HF with it later then you will need a current balun (choke) at the feed point of the Antenna to isolate the coax from the antenna and reduce common mode interference on the coax braid, and i would suggest investing in a NanoVNA (about £40 or less on ebay) they make tuning an antenna so much easier (download the free VNAsaver software as well, makes them much more user friendly)

if you are planning to explore digital modes (FT8, RTTY, PSK, SSTV) you will likely need one of the free "virtual audio cable" programs to take the audio from the SDR program (SDR#, Cubic SDR etc) and feed into WSJT-x (FT8/4, wspr, MSK144 etc) , Yoniq / MMSSTV for SSTV, or FLDigi for (RTTY, CW reader, PSK etc)

also check what the lowest frequency the SDR dongle can go, as many cut off at about 25mhz, you can get down convertors but make sure you get one with a TCXO and drift of less than 1ppm or you will find it drifts all over the place when you try to listen to digital mode transmissions, especially wspr and QRSS which are very narrow bandwidth and need stable reciever frequency to decode them

if you are planning to listen to digital modes, make sure the time is accurate on your PC, goto Time.is and it will tell you how far out your PC clock is, for digital modes you need to be within 1 second or you won't decode the signal on FT8/4 / WSPR etc, (RTTY / PSK31 / SSTV don't care about time)

the usual bandwidth depends on the modulation type, AM is about 6Khz, SSB is about 2.7-3Khz, digital modes use a fraction of an SSB channel, FT8/4 use 50hz slots, WSPR uses about 10hz slots, QRSS is about 5hz, CW is just the carrier (on/off), FM is only used 10m (28Mhz) and up, start getting used to looking at the bandplan 14MHz - Radio Society of Great Britain - Main Site : Radio Society of Great Britain – Main Site (rsgb.org) as it will give you a clue where to look for the type of signal / mode you are looking for

if you are looking for a cheaper end HF radio, look at the Xiegu G90 (<£400 new for 20w all mode 160m-10m rig, good built in ATU, and a good support group on groups.io), i use one here along with an Yaesu FT991a, you can get the interfaces for digital for the G90 for about £60 or less, 

As a new M7 on HF with 10w you will struggle on voice, digital modes are easiest to get going, 10w on FT8 you will work most of EU, some east coast US and some south america, you will also be able to see SSTV on 20m (14.230), RTTY / PSK31 around 14.070, 7.040-7.050 to name a few modes and frequencies

don't forget with a PC and a ham license you can also use echolink to get around the world to amateurs out of your normal range using internet connected repeaters

get yourself set up on QRZ.com and eQSL.cc means you can start logging and collecting QSL cards for your contacts, both can also import the ADIF log files from WSJT-x and FLDigi which means easy logging, or there are paid for things like HRD (ham radio deluxe) that can automate logging as well

and finally don't forget the RSGB do a series of video's Foundation Practicals – video guides - Radio Society of Great Britain - Main Site : Radio Society of Great Britain – Main Site (rsgb.org)



(not pete who runs the site, he is M0PSX)

Phil M7PHX
7 months ago

Hi Peter,

thanks very much for your very detailed reply!

The Dongle @ <1ppm, QRZ, Echoham and a Balun as centre feed, I already have, so that's good. I've seen the Xiegu G90 on youtube and it seems like you got a lot of bang for your buck, good to know that you're using one fine.

The digital modes seem quite prevalent on 20m, more than analogue voice. I'll have a look into the decoding software. Piping the audio with a virtual audio cable into a separate piece of software I didn't know. I assumed there would be plugins within Cubic, SDR sharp etc software to do it directly.

There's plenty to go and check out. thanks again Phil

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