Why can't I transmit on some VHF channels?

[whipper_snapper]

Re: Why can\'t I transmit on some VHF channels?

[ QUOTE ]
it is not the cutting off that is important, it is the overall length of the cable. By adding 5m you have changed the length. If you understand frequencies have wavelengths, then try to imagine for simplicity, the sine waves travel up the cable to the aerial, added to this complication you get a reflected wave back from the aerial as no aerial will ever transmit 100% of the forward wave, this is the SWR, a reflected wave can cancel out the 'good' wave travelling up from the radio. You can not remove the reflected wave, but you can reduce it's affect by balancing the cable length and thus the point at which the reflection hits the forward wave. This is what you have done be changing the cable length.


[/ QUOTE ]

Better get up into the loft.

 

[ShipsWoofy]

Re: Why can\'t I transmit on some VHF channels?

fine, last time I try to explain something simply for non-techs to try and get a grip of what is happening.

This is why many people allow electrical questions to disappear without answer, always some smartarse to put them down.

If you think you can do better go ahead, rather than mocking those who try.

Yes, you have pi$$ed me off.

 

[Anonymous]

Re: Why can\'t I transmit on some VHF channels?

[ QUOTE ]

Better get up into the loft.

[/ QUOTE ]Eh? I thought that DogWatch gave a pretty excellent explanation. What's your explanation?

 

[whipper_snapper]

Re: Why can\'t I transmit on some VHF channels?

Errrm, hang on, I will just nip up into the loft!


But seriously, the length of a balanced coax feed is not critical, is it ? I have certainly never treated it as such in dozen's of installations at VHF and UHF and never had any problems, and always measure SWR after each installation.

 

[Heckler]

Re: Why can\'t I transmit on some VHF channels?

Jools
As I understand it, the coax plays no part in the "tuned" length, so in this case it should be irrelevant. The whip is the "exposed" bit (simplisticly speaking) I suspect that the "joints" have opened a part of the circuit to allow signals to "escape" or somesuch strangeness.
Let us not forget that the coax isnt conducting voltage as such, rather the inner insulation acting as a "waveguide space" and the outer braid serving to keep the signals moving along. Perhaps a better analogy would be that light is being shone along a tube with the outer braid being the tube and the inner copper strand the inner part of the tube, the light would then be a tube with the braid as the outside and the inner strand being straddled by the tube of signals.
stu

 

[Anonymous]

Re: Why can\'t I transmit on some VHF channels?

It's not balanced and the assumption is that there is a mismatch.

Have a look at the Glomex instructions....they all mention the possible need to vary length.

In a well-matched system length, of course, would not matter but these aerials are pretty frequency dependant and the connectors are really crappy. It's hard to get better than 1.5:1 /forums/images/graemlins/shocked.gif And that's with care, and as an experienced wireman.

 

[whipper_snapper]

Re: Why can\'t I transmit on some VHF channels?

OK, I can see that if there is a serious mismatch then length becomes critical, but that means that the problem has not been fixed at all just the symptoms minimized. I would want to look at the cause of the mismatch.

Sorry dogwatch, my flippant remark was not mean to sound as condescending as it appears.

Time for bed!

 

[ShipsWoofy]

Re: Why can\'t I transmit on some VHF channels?

Wow, you would be getting into waveguide theory then, I left waveguide behind a long time ago.

I detested that stuff, a dimple here, a slug there, hated hated hated.

I was just attempting a simple analogy of why length is an important part of an RF circuit. As I said at the bottom, considering the faults he describes I would suggest renewing the whole cable.

whipper, apology accepted, sorry too for throwing my teddy, I rarely answer or even venture into these threads for the reason I stated. It is a very complex field and I have always tried to write in a way to help people get a grip, not turn them into design engineers. I studied for 3.5 years in RF, I tend to leave out little bits of that when I shove it into a paragraph.

 

[Gubbo]

Re: Why can\'t I transmit on some VHF channels?

Thank-you for the explainations. The cable length changes made an immediate and measurable difference to the performance. Your reflected wave post made sense to me as a layman.

Fully appreciate it's very difficult to diagnose someone else's practical problems from afar... which is why I originally asked a kind-of theoretical question about why a radio should transmit on some channels and not on others.

In light of a few posts still appearing, I must clarify that, in working through this, I have replaced all the cable inside the boat with new. There is none of the old blackened stuff left....but that didn't solve the problem in any case. The mast run is only 6 months old as is the antenna. All cable is RG58 and uses proper connectors. The in-boat part was much older than the mast bit because I didn't chnage that when I fitted the new antenna last summer.

Having changed the length of the cable... The radio now works on all channels.

You suggest changing the cable still. OK... willing to so if this means an even better system... but what do you think may be the problem with the current stuff? What am I looking for?

I agree that a meter would be useful so I can see how efficient the system is. Perhaps I need to ask around the marina armed with a bottle of wine as suggested near the beginning of this post.

 

[Anonymous]

Re: Why can\'t I transmit on some VHF channels?

[ QUOTE ]

You suggest changing the cable still. OK... willing to so if this means an even better system... but what do you think may be the problem with the current stuff? What am I looking for?

I agree that a meter would be useful so I can see how efficient the system is. Perhaps I need to ask around the marina armed with a bottle of wine as suggested near the beginning of this post.

[/ QUOTE ]PhotoDog has been doing a grand job communicating a very technical subject in a way that is easy to understand, so I hesitated before jumping in but.......

You sound like someone who is not fazed by technology and you can learn from instructions. There is no reason why you cannot learn to use a 'meter' correctly. A meter should cost you between £40 to £60 delivered with the bits you need. You've then got that for life and can check....

a) That the radio is pushing out its full 25W
b) That the aerial is actually launching that RF energy into the great outdoors and not just reflecting it back into the set, cooking the power transistors.

It's certainly worth checking once or twice a season. I've been borrowing up to now and it is a real pain. I am about to buy. Richard recently posted a link to a suitable meter http://www.rocketradio.co.uk/avair-av-40-140-525mhz-cross-needle-swr--power-meter-1242-p.asp I am trying to order one (for delivery to a UK address) and am struggling to communicate with them but the product looks good. You will need patch leads, and, I suggest, a dummy load as well. If you want to go down that road, when mine is finally on order (today, I hope) I will post the parts list and prices.

 

[Bilgediver]

Re: Why can\'t I transmit on some VHF channels?

As I understand it, the coax plays no part in the "tuned" length,
**************************************************


Theoretically what you say is correct.....Practically it is not as this poster has found.....To satisfy your self get several marine antennas and attach them to a vhf with an SWR meter in circuit. check the swr then cut of coax at about 2" a time and retest and you will be amazed at what you see. If a manufacturer puts a reverse current detector in his circuit I am not surprised some radios do as this one did. It suggests that many marine VHF antennas are not 50ohm!!!!

I have improved the transmit performance of many boats over the years by cutting the coax to make the transmitter think it sees a better sytem. It certainly worked for the boat owners concerned. Yes it sounds white mans magic but if it works don't knock it! /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif

 

[Gubbo]

Re: Why can\'t I transmit on some VHF channels?

After all the success, I tried it on my emergency set-up and the dreaded ANT returned. Fo ref this is a brand new VHF antenna connected via 4m of cable using PL soldered connectors.

I've spoken to simrad and they suggested I sent it back to them for checking out. I've done so and await their verdict.