Swr Readings

I set up a new outfit a week or so ago and temporarily set up the  new silver rod , half wave . on a short aluminium pole about 3ft off the ground to try it . The radio is programmed with band A - E basically divided into 40 channels   from frequency 25.625 through to 27.99125 which includes the UK CB at the upper end . The SWR readings were very low on most frequencies . no more than about 1.5 at the highest.
I moved the aerial to it's permanent fixture on the side of the garage on an  aluminium pole . the base of the silver rod is clear of the roof and about 9 to 10 ft off the ground.
The SWR has gone up slightly . These were the readings I got.

Band A Ch1 (25.615 ) 1.9 Ch40 (26.055)  both  swr 1.9.

Band B Ch1  (26.065)  swr 2.0  Ch40 (26.505) swr 1.5

UK / CB  Ch 40 swr of 1.5 & Ch1 swr   1.3 .

The mid band (26.965 to 27.405 mhz) all frequencies were about swr  1.2.

It seems the  lower end below 26 mhz is the highest reading .

Do you think it's worth faffing with taking the pole down and trying to lower the readings ?
Rich.

I would, your antenna is a tad too short, the lower the frequency, the longer the antenna needs to be.
Lower the antenna and extend the top section or any section by a inch for starters, then put the ant back up again to check the Vswr.
Good luck.

Good luck.

Rjbingham wrote:

I would, your antenna is a tad too short, the lower the frequency, the longer the antenna needs to be.
Lower the antenna and extend the top section or any section by a inch for starters, then put the ant back up again to check the Vswr.
Good luck.

Good luck.

Thanks for that . I'll give it a go . . It's a nice day for it now .

Since the last post the weather faired up. The above readings were taken this morning in heavy drizzle , would this affect the readings ?  It's now warm and sunny , and now, on channel 1  band A (25.615 mhz)  the radio gives the Hi swr alarm and ch40 in the same band  (26.055 mhz) is reading over 2.0 . the other bands above this are OK mostly .
Rich'

Hi Rich. Changing mount height of antenna above ground can affect SWR. Weather can do a little too sometimes.. my antennas are inside in my loft and so stay bone dry, but I notice the SWR drops slightly when it's raining.

What i do is try to set the SWR to it's lowest around where in the band i intend to use it most.
I posted a picture this morning in the antenna section with a 1 to 1 SWR on 27.550MHz but that was not the reason i posted it really if you look at the post.

I tried lengthening the aerial an inch or so  . I've got it so that it's not sounding the alarm on the lowest frequency (25.615 mhz) now , but it's still reading over 2 , extending the length further seems to make the S hi alarm sound again .
up around 26.285mhz the reading is just over 1.5 and seems to keep getting lower as you increase the frequency . in the mid bands over 27 mhz mostly the needle barely moves and  never above 1.5 . 
I assume I am doing the right thing by having the radio knob set at full power while checking the readings across the spectrum and noting the frequencies rather than channel numbers  ?

Changing the length of you coax can affect the Vswr as well, your on the right track, try to find a sweet spot for the lowest frequency where your not tripping the rigs alarm and maybe add a length of coax to see if that helps, about 6 to 8 feet.
Ady

Rjbingham wrote:

Changing the length of you coax can affect the Vswr as well,

It absolutely should not. If changing the length of your coax affects the SWR it shows that your coax is being used by the antenna system to compensate for an inadequate RF ground so therefore changing the length of the coax alters the tuning of the antenna.

That also means common mode RFI which can mean increased noise on receive, noise in your audio on transmit as well as causing interference to others and your own equipment. In extreme cases when running power it can also cause mike bite, a tingling you get when holding and talking into the mike which is actually RF burns.

Rich13 wrote:

Since the last post the weather faired up. The above readings were taken this morning in heavy drizzle , would this affect the readings ?

Yes. RF flows over the surface of a conductor not through it. Anything on the surface of that conductor affects what is called the velocity factor of it and can make it appear electrically longer than it is. Rain will do that.

I wouldn't personally give a toss what the SWR is on the lowest frequencies because sod all people use anything below 26MHz. I would concentrate on getting the low centred around 27.555. That'll give you useable SWR down to 26MHZ and should also give you coverage of a decent portion of the amateur radio 10m band at 28MHz and above you could use should you decide to take your licence. DO NOT USE ANY FREQUENCY ABOVE 27.999MHZ IF YOU DON'T HAVE A HAM LICENCE.

Having had only  receivers over the last 40 years or so  I'm just finding my way round the bands I can transmit on now. and it does seem most of what I've heard has been on the mid bands over 27mhz where it appeared busy the day I originally set up , and my  readings are  good there any way  . It's been quiet  the last day or so mind you which I assume is down to the conditions?

Rjbingham wrote:

Changing the length of you coax can affect the Vswr as well

Small differences in coax should make no noticeable difference to SWR, and when it does you have a problem.

Its typically a common mode currents issue from lack of a proper ground plane, although there are one or two other things that can cause this as well.  Magnet mounts on vehicles, and base antennas with no ground planes tend to have this issue the most.

if adding or removing a few feet of coax makes any difference to your SWR, you should fix the problem causing it, then re-tune the antenna.  Likely a well placed choke on the feed line will be enough.


The DB

Rich13 wrote:

Having had only  receivers over the last 40 years or so  I'm just finding my way round the bands I can transmit on now. and it does seem most of what I've heard has been on the mid bands over 27mhz where it appeared busy the day I originally set up , and my  readings are  good there any way  . It's been quiet  the last day or so mind you which I assume is down to the conditions?

Yep. Band conditions can vary between very good where you'll hear people from thousands of miles away to so bad it can have you checking that you've still got an antenna. I remember once being out in my car on the 20m ham band and it just went stone cold dead in a matter of seconds. I got out my car, checked everything, even swapped the radio for another I happened to have in the car at the time. When I got home I went online and found the cause when I read the solar weather reports. Bloody sun turned HF off.

Learn to read solar reports. http://www.hamqsl.com/solar.html has them free to view. If you look on the left of that site you'll see a table. To start off with concentrate on three figures, SFI, K and MUF. SFI stands for Solar Flux Index. The higher it is the better the conditions on higher frequencies. >100 is good for DXing for the 11m band, >150 it'll be really rocking. Conversely the lower it is the better it is on low bands. Sub 60 and the 80m ham band will be rocking. The K index is the next. The higher the number the more "switched off" HF is. An ideal is 1. By the time it gets to 3 you're not going to be hearing much. 4 or 5 and you'll be looking to see if you've still got an antenna attached. The A index is an average of the K index over time and used to give you an indication of whether things are improving or getting worse. Finally MUF or Maximum Usable Frequency. This gives a guide to the highest frequencies in MHz that are likely to benefit from solar propogation.

If that is too much to take in the chart handily also includes a table that breaks down the conditions for ranges of the HF amateur bands so for 11m take note of the 12m-10m one. However don't take that as a be all and end all because 11m can also benefit from other types of propogation and often you can still make long distance contacts when the solar reports aren't that favourable. But at least now you have a resource where you can maybe find a cause of why today you're not hearing much DX when everything appears to be working fine.

I just love and appreciate the knowledge you guys have and share for free. Thanks to all of you.

https://www.solarham.net/

Northern Crusader wrote:

If that is too much to take in the chart handily also includes a table that breaks down the conditions for ranges of the HF amateur bands so for 11m take note of the 12m-10m one. However don't take that as a be all and end all because 11m can also benefit from other types of propogation and often you can still make long distance contacts when the solar reports aren't that favourable. But at least now you have a resource where you can maybe find a cause of why today you're not hearing much DX when everything appears to be working fine.

Thank you for the detailed explanation. Just been looking at the propagation tables.
I used to do  some  listening to the HF oceanic air bands and recall that they had primary and secondary frequencies issued on the day to cope with conditions quite often during the day they would be in the 5 - 13 occasionally up to 17 mhz range and during the hours of darkness they would often use 3-4 mhz  .  Having access for the moment to one band width , obviously you just have to capitalise on the good days . I was fortunate enough to actually get a brief contact with a couple of continental stations while I was testing my new set up . Glad I didn't do it the following day or I'd have spent the last few days "fault finding" .

Nothing wrong with them SWR readings , crack on and enjoy



Craig CT Admin 73

Just been looking at the propagation tables.
I used to do  some  listening to the HF oceanic air bands and recall that they had primary and secondary frequencies issued on the day to cope with conditions quite often during the day they would be in the 5 - 13 occasionally up to 17 mhz range and during the hours of darkness they would often use 3-4 mhz.

Rich, thats simply because you have certain waypoints to report in-flight, meaning at a certain point you call-up on HF ( as VHF voice comms are not viable in certain areas) to report you are passing 'said' waypoint, this way air-traffic know where you are and they know you have not ditched into the atlantic for example.
All the frequencies are pre-stored into the comms sets and you are allocated a 'selcall' a unique number to identify yourself when asked.
Civil aircraft and the military work virtually the same way.
You still have a Satellite connection which updates your position, but as MH370 showed, its not foolproof !!

Ady

Cheers for that information , and your comment about the frequency range  the antenna has to cover makes sense when put like that .I suppose you will only be able to optimise it in a fairly narrow range preferably the ones used most , which I think I have now .

Rich,
My 160m vertical (1.8mhz)has a bandwidth of 50khz.!!!!
Which is enough cover the CW part of the band here, it has a very 'Q', that means it rejects signals outside of the band.
A large bandwidth with a low Vswr involves making compromises in performance.