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Signal loss - too long coax?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by M0FVD, Jun 16, 2021.

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  1. M0FVD

    M0FVD Ham Member QRZ Page

    Hello all

    Because of the layout of the house and garden my shack is approximately 30meters of RG213 away from antenna (a colinear due to lack of space for a yagi & rotator) with Ntype connectors. I have three units (FT847, FRG9600 and FT902DM with FTV901) and I get poor reception on all devices at 2m and 70cm even from relatively local repeaters!

    Am I.pushing my luck at 30m length of half-decent coax? Could a preamp help? Any recommendations?
    Any help at all? Thanks in advance
  2. W4HWD

    W4HWD Premium Subscriber QRZ Page

    According to an online loss calculator, a 30 meter run of RG-213 has the following loss:

    144MHz: -2.55 dB

    440MHZ: -4.85 dB

    Thats alotta loss.

    Switching to 30 meters of LMR-400 would be an improvement:

    144MHz: -1.47 dB

    440MHz: -2.6 dB

    It would be worth it to switch.
  3. M0FVD

    M0FVD Ham Member QRZ Page

    That's a handy comparison. Thank you.

    So at 70cm with LMR-400 that's 2.25dB up against RG214
    3 which if memory is correct (and probably isn't!) that's like an improvement in signal of about a half of it again?

    If I'm on the right lines that's a big improvement and could make a noticeable difference.

  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    I would start in the other end, checking the antenna and its noise environment first before changing the cable.

    A 2 dB difference between cables is not significant enough to make a marked difference in signal strength, especially when receiving FM above the detection threshold.

    Also, there may be local noise that is masking your signals.

    Unfortunately, there are many noise sources around in the urban environment today which reach up even to 432 MHz.

    It is not very uncommon that local noise mask the receiver thermal noise level with 20 dB or more nowadays. In these cases, the cable attenuation in the receive direction is quite moot.

  5. M0FVD

    M0FVD Ham Member QRZ Page

    That's also an interesting idea: we have a lot of electrical noise from chargers and smart-home devices etc. I've come back to radio after a gap of about 30 years and staggered by the noise on the bands now. Much of HF is unusable to me here near the city.
  6. W9IQ

    W9IQ Ham Member QRZ Page

    You should also measure the SWR or complex input impedance of your antenna on the two bands. One reason is to see if there is a problem with the antenna and the other is to include the additional coax loss due to elevated SWR in your antenna system estimates.

    For a given antenna on VHF and UHF, increasing the antenna height is nearly always the dominant factor in increasing your receive and transmit signal strength.

    - Glenn W9IQ
    W7PDK likes this.
  7. KI8DJ

    KI8DJ Ham Member QRZ Page

    Gain of the collinear largely offsets the loss I don't think you would gain much performance for the cost. Hardline would be real pricey.
  8. SM0AOM

    SM0AOM Ham Member QRZ Page

    I have recently researched this very subject in connection with evaluating the risk of interfering with air traffic control and public safety communications in the VHF and UHF bands by primarily electronic appliances and solar panels.

    The results were that wide-band interference levels in urban and sub-urban areas of more than 20 dB above the thermal noise floor are quite common, and that the levels are increasing.

    Suggest that you use a portable receiver, preferably with AM detection, and connect to the antenna with the shortest possible feed-line. If the noise level increases when the antenna is connected, the system is external noise limited.

  9. WA9PND

    WA9PND Ham Member QRZ Page

    What Karl-Arne said. If the system is not limited by external noise then a preamp could help - but the preamp would need to be at the antenna, not at the rig.
  10. WN1MB

    WN1MB Ham Member QRZ Page

    Right. And remember that pre-amps amplify noise, too.

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